[msan] Fix bugs when instrument x86.avx512*_cvt* intrinsics.

Scalar intrinsics x86.avx512*_cvt* have an extra rounding mode operand.
We can directly ignore it to reuse the SSE/AVX math.
This fix the bug https://bugs.llvm.org/show_bug.cgi?id=48298.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D92206