An x86 function returns a floating point value in st(0), and we must make sure

it is popped, even if it is ununsed. A CopyFromReg node is too weak to represent
the required sideeffect, so insert an FpGET_ST0 instruction directly instead.

This will matter when CopyFromReg gets lowered to a generic COPY instruction.

llvm-svn: 108037