[msan] Combine shadow check of the same instruction

Reduces .text size by 1% on our large binary.

On CTMark (-O2 -fsanitize=memory -fsanitize-memory-use-after-dtor -fsanitize-memory-param-retval)
Size -0.4%
Time -0.8%

Reviewed By: kda

Differential Revision: https://reviews.llvm.org/D133071