PPC: Prep for if conversion of bctr[l]

This adds in-principle support for if-converting the bctr[l] instructions.
These instructions are used for indirect branching. It seems, however, that the
current if converter will never actually predicate these. To do so, it would
need the ability to hoist a few setup insts. out of the conditionally-executed
block. For example, code like this:
  void foo(int a, int (*bar)()) { if (a != 0) bar(); }
becomes:
        ...
        beq 0, .LBB0_2
        std 2, 40(1)
        mr 12, 4
        ld 3, 0(4)
        ld 11, 16(4)
        ld 2, 8(4)
        mtctr 3
        bctrl
        ld 2, 40(1)
.LBB0_2:
        ...
and it would be safe to do all of this unconditionally with a predicated
beqctrl instruction.

llvm-svn: 179156