[PowerPC] Change liveness testing in VSX FMA mutation pass

With VSX enabled, LLVM crashes when compiling
test/CodeGen/PowerPC/fma.ll.  I traced this to the liveness test
that's revised in this patch. The interval test is designed to only
work for virtual registers, but in this case the AddendSrcReg is
physical. Since there is already a walk of the MIs between the
AddendMI and the FMA, I added a check for def/kill of the AddendSrcReg
in that loop.  At Hal Finkel's request, I converted the liveness test
to an assert restricted to virtual registers.

I've changed the fma.ll test to have VSX and non-VSX variants so we
can test both kinds of multiply-adds.

llvm-svn: 220090