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.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <cstdint>
+
+#if defined(__x86_64__) && !defined(USE_SIMD_FALLBACK)
+
+// SIMD operations on potentially-indeterminate Node16::index[16] bytes.
+// Implemented in file-level assembly (simd_x86_64.S) because loading and
+// operating on indeterminate values is UB in C++ but well-defined in
+// assembly. The caller must mask the returned bitfield to
+// [0, numChildren) before using it.
+//
+// Each function returns a 16-bit bitmask in the low 16 bits of a uint32_t
+// (upper 16 bits are zero). Bit i is set iff the condition holds at index i.
+
+extern "C" {
+// Returns bit i set iff idx[i] == key
+uint32_t find_eq_16(const uint8_t idx[16], uint8_t key);
+// Returns bit i set iff idx[i] >= child
+uint32_t find_ge_16(const uint8_t idx[16], uint8_t child);
+// Returns bit i set iff begin <= idx[i] < end
+uint32_t mask_in_range_16(const uint8_t idx[16], uint8_t begin, uint8_t end);
+}
+
+#endif