[llvm-exegesis] X87 RFP setup code.

Summary:
This was lost during refactoring in rL342644.

Fix and simplify simplify value size handling: always go through a 80 bit value,
because the value can be 1 byte). Add unit tests.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D53423

llvm-svn: 344779