[X86] Fix register class for assembly constraints to ST(7). NFCI.

Modify getRegForInlineAsmConstraint to return special singleton
register class when a constraint references ST(7) not RFP80 for which
ST(7) is not a member.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351206 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed