[llvm-exegesis][x86] Add option to prevent use of xmm8-xmm15 upper SSE registers

[llvm-exegesis][x86] Add option to prevent use of xmm8-xmm15 upper SSE registers

Noticed while trying to use llvm-exegesis to get some accurate capture numbers on some old Atom/Silverment hardware as part of the work with D103695.

These targets' frontends are particularly poor and the use of the xmm8-xmm15 SSE registers results in longer instruction encodings which were affecting the latency/throughput estimates.

Thanks to @lebedev.ri for the --skip-measurements command line argument which made testing much easier!

Differential Revision: https://reviews.llvm.org/D138832