[llvm-mca][X86] Teach how to identify register writes that implicitly clear...

[llvm-mca][X86] Teach how to identify register writes that implicitly clear the upper portion of a super-register.

This patch teaches llvm-mca how to identify register writes that implicitly zero
the upper portion of a super-register.

On X86-64, a general purpose register is implemented in hardware as a 64-bit
register. Quoting the Intel 64 Software Developer's Manual: "an update to the
lower 32 bits of a 64 bit integer register is architecturally defined to zero
extend the upper 32 bits".  Also, a write to an XMM register performed by an AVX
instruction implicitly zeroes the upper 128 bits of the aliasing YMM register.

This patch adds a new method named clearsSuperRegisters to the MCInstrAnalysis
interface to help identify instructions that implicitly clear the upper portion
of a super-register.  The rest of the patch teaches llvm-mca how to use that new
method to obtain the information, and update the register dependencies
accordingly.

I compared the kernels from tests clear-super-register-1.s and
clear-super-register-2.s against the output from perf on btver2.  Previously
there was a large discrepancy between the estimated IPC and the measured IPC.
Now the differences are mostly in the noise.

Differential Revision: https://reviews.llvm.org/D48225

llvm-svn: 335113