Don't clobber pending ST regs when FP regs are killed.

X86FloatingPoint keeps track of pending ST registers for an upcoming
inline asm instruction with fixed stack register constraints.  It does
this by remembering which FP register holds the value that should appear
at a fixed stack position for the inline asm.

When that FP register is killed before the inline asm, make sure to
duplicate it to a scratch register, so the ST register still has a live
FP reference.

This could happen when the same FP register was copied to two ST
registers, or when a spill instruction is inserted between the ST copy
and the inline asm.

This fixes PR10602.

llvm-svn: 137050