[regalloc] Make RegMask clobbers prevent merging vreg's into PhysRegs when hoisting def's upwards.

Summary:
This prevents vreg260 and D7 from being merged in:
  %vreg260<def> = LDC1 ...
  JAL <ga:@sin>, <regmask ... list not containing D7 ...>
  %D7<def> = COPY %vreg260; ...
Doing so is not valid because the JAL clobbers the D7.

This fixes the almabench regression in the LLVM 3.7.0 release branch.

Reviewers: MatzeB

Subscribers: MatzeB, qcolombet, hans, llvm-commits

Differential Revision: http://reviews.llvm.org/D11649

llvm-svn: 243745