Convert uses of getPhysicalRegisterRegClass in VirtRegRewriter.cpp.

The first one was used just to call isSafeToMoveRegClassDefs. In
general, using a more specific reg class is better, in practice only
x86 implements that method and the results are always the same.

The second one is in FindFreeRegister and is used to check if a register
is in a register class, a much more direct call to contains is better as
it should cover more cases and is faster.

llvm-svn: 108093