[llvm-exegesis] Allow the target to disable the selection of some registers.

Summary:
This prevents "Cannot encode high byte register in REX-prefixed instruction"
from happening on instructions that require REX encoding when AH & co
get selected.
On the down side, these 4 registers can no longer be selected
automatically, but this avoids having to expose all the X86 encoding
complexity.

Reviewers: gchatelet

Subscribers: tschuett, jdoerfert, llvm-commits, bdb

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59821

llvm-svn: 357003