Don't emit st(0)/st(1) copies as FpMOV instructions. Use FpSET_ST? instead.

Based on a patch by Rafael Espíndola.

Attempt to make the FpSET_ST1 hack more robust, but we are still relying on
FpSET_ST0 preceeding it. This is only for supporting really weird x87 inline
asm.

We support:

  FpSET_ST0
  INLINEASM

  FpSET_ST0
  FpSET_ST1
  INLINEASM

with and without kills on the arguments. We don't support:

  FpSET_ST1
  FpSET_ST0
  INLINEASM

nor

  FpSET_ST1
  INLINEASM

Just Don't Do It!

llvm-svn: 108047