Fix a bug in the -regalloc=fast handling of exotic two-address instruction with

multiple defs, like t2LDRSB_POST.

The first def could accidentally steal the physreg that the second, tied def was
required to be allocated to.

Now, the tied use-def is treated more like an early clobber, and the physreg is
reserved before allocating the other defs.

This would never be a problem when the tied def was the only def which is the
usual case.

This fixes MallocBench/gs for thumb2 -O0.

llvm-svn: 109715