Switch ARM target to register masks.

I'll let the buildbots determine the compile time improvements from this
change, but 464.h264ref has 5% faster codegen at -O2.

This patch does cause some assembly changes.  Branch folding can make
different decisions about calls with dead return values.
CriticalAntiDepBreaker may choose different registers because its
liveness tracking is affected.  MachineCopyPropagation may sometimes
leave a dead copy behind.

llvm-svn: 151331