Remove UB from indeterminate value handling

Move SIMD operations on potentially-indeterminate Node16::index bytes
into file-level assembly, where loading and operating on indeterminate
values is well-defined (unlike C++). Restructure scalar fallback loops
to iterate [0, numChildren) instead of [0, kMaxNodes). Fix TrivialSpan
construction from indeterminate pointers in check::Job::init and
insertPointWritesOrSorted to only construct when end.len > 0.

Add MSan toolchain to the debug CI build to catch these issues going
forward.

simd_x86_64.S

@@ -0,0 +1,74 @@
+// SIMD operations on potentially-indeterminate Node16::index[16] bytes.
+// Written in assembly so msan doesn't track the loads. The caller is
+// responsible for masking the returned bitfield to [0, numChildren) before
+// using it.
+//
+// All functions return a 16-bit bitmask in %eax (bit i set = condition true
+// at index i). The upper 16 bits of %eax are zero.
+//
+// System V AMD64 ABI:
+//   %rdi = const uint8_t *idx  (16 bytes)
+//   %esi = uint8_t key         (find_eq_16, find_ge_16)
+//   %sil = uint8_t begin       (mask_in_range_16)
+//   %dl  = uint8_t end         (mask_in_range_16)
+
+    .text
+
+// uint32_t find_eq_16(const uint8_t idx[16], uint8_t key)
+//   Returns bit i set if idx[i] == key
+    .globl find_eq_16
+    .type  find_eq_16, @function
+find_eq_16:
+    vmovd      %esi, %xmm1          // broadcast key
+    vpbroadcastb %xmm1, %xmm1
+    vmovdqu    (%rdi), %xmm0        // load 16 bytes (may contain indeterminate data)
+    vpcmpeqb   %xmm0, %xmm1, %xmm0  // 0xff for each match
+    vpmovmskb  %xmm0, %eax          // 16-bit bitmask
+    movzwl     %ax, %eax            // zero-extend to 32 bits
+    ret
+    .size find_eq_16, .-find_eq_16
+
+// uint32_t find_ge_16(const uint8_t idx[16], uint8_t child)
+//   Returns bit i set if idx[i] >= child
+//   x86 doesn't have a "compare unsigned >=" for bytes directly, so we use:
+//   min(key, idx[i]) == key  iff  idx[i] >= key
+    .globl find_ge_16
+    .type  find_ge_16, @function
+find_ge_16:
+    vmovd      %esi, %xmm1
+    vpbroadcastb %xmm1, %xmm1       // key broadcast
+    vmovdqu    (%rdi), %xmm0        // load 16 bytes
+    vpminub    %xmm0, %xmm1, %xmm2  // min(key, idx[i])
+    vpcmpeqb   %xmm2, %xmm1, %xmm0  // 0xff where min == key, i.e. idx[i] >= key
+    vpmovmskb  %xmm0, %eax
+    movzwl     %ax, %eax
+    ret
+    .size find_ge_16, .-find_ge_16
+
+// uint32_t mask_in_range_16(const uint8_t idx[16], uint8_t begin, uint8_t end)
+//   Returns bit i set if begin <= idx[i] < end
+//   Logic: (idx[i] - begin) < (end - begin)  [unsigned wrapping arithmetic]
+//   Equivalently: idx[i] - begin != max(idx[i] - begin, end - begin)
+//   i.e. idx[i] - begin is NOT equal to the saturated value.
+//   We compute: sub = idx - begin; result = (sub < (end-begin)) for each byte.
+//   Using: sub == max(sub, end-begin) means NOT in range.
+//   So: in_range = ~(movemask(cmpeq(sub, max(sub, range_size))))
+    .globl mask_in_range_16
+    .type  mask_in_range_16, @function
+mask_in_range_16:
+    vmovd      %esi, %xmm1          // begin
+    vpbroadcastb %xmm1, %xmm1
+    vmovd      %edx, %xmm2          // end
+    vpbroadcastb %xmm2, %xmm2
+    vmovdqu    (%rdi), %xmm0        // load 16 bytes
+    vpsubb     %xmm1, %xmm0, %xmm0  // idx - begin (wrapping)
+    vpsubb     %xmm1, %xmm2, %xmm2  // end - begin (range size)
+    vpmaxub    %xmm0, %xmm2, %xmm3  // max(idx-begin, range_size)
+    vpcmpeqb   %xmm3, %xmm0, %xmm0  // 0xff where NOT in range
+    vpmovmskb  %xmm0, %eax
+    not        %eax                 // invert: 1 = in range
+    movzwl     %ax, %eax
+    ret
+    .size mask_in_range_16, .-mask_in_range_16
+
+    .section .note.GNU-stack,"",@progbits