Fix a bug in the calculation of the VEX.B bit for FMA4 rr with the VEX.W bit set. The VEX.B was being calculated from the wrong operand. Fixes at least some portion of PR14185.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177014 91177308-0d34-0410-b5e6-96231b3b80d8