Codegen FP adds and subtracts with a constant more efficiently, generating:

        fld QWORD PTR [%ESP + 4]
        fadd QWORD PTR [.CPItest_add_0]

instead of:

        fld QWORD PTR [%ESP + 4]
        fld QWORD PTR [.CPItest_add_0]
        faddp %ST(1)

I also intend to do this for mul & div, but it appears that I have to
refactor a bit of code before I can do so.

This is tested by: test/Regression/CodeGen/X86/fp_constant_op.llx

llvm-svn: 12839