[CriticalAntiDepBreaker] Teach the regmask clobber check to check if any subregister is preserved before considering the super register clobbered
X86 has some calling conventions where bits 127:0 of a vector register are callee saved, but the upper bits aren't. Previously we could detect that the full ymm register was clobbered when the xmm portion was really preserved. This patch checks the subregisters to make sure they aren't preserved.
Differential Revision: https://reviews.llvm.org/D70699