Handle early-clobber registers in the aggressive anti-dep breaker

The aggressive anti-dep breaker, used by the PowerPC backend during post-RA
scheduling (but is available to all targets), did not handle early-clobber MI
operands (at all). When constructing the list of available registers for the
replacement of some def operand, check the using instructions, and remove
registers assigned to early-clobbered defs from the set.

Fixes PR21452.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223727 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r223709, "[PowerPC]Activate FeatureVSX for the Power target", to unbreak bots.

CodeGen/PowerPC/vsx-p8.ll was failing.

  '+power8-vector' is not a recognized feature for this target (ignoring feature)
  llvm/test/CodeGen/PowerPC/vsx-p8.ll:33:14: error: expected string not found in input
  ; CHECK-REG: lxvw4x 34, 0, 3
               ^
  <stdin>:50:2: note: scanning from here
   .align 3
   ^
  <stdin>:61:2: note: possible intended match here
   lvx 3, 0, 3
   ^

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223729 91177308-0d34-0410-b5e6-96231b3b80d8