Simplify code by using the more powerful BuildMI forms.

Implement a small optimization.  In test/Regression/CodeGen/X86/select.ll,
we now generate this for foldSel3:

foldSel3:
        mov %AL, BYTE PTR [%ESP + 4]
        fld DWORD PTR [%ESP + 8]
        fld DWORD PTR [%ESP + 12]
        mov %EAX, DWORD PTR [%ESP + 16]
        mov %ECX, DWORD PTR [%ESP + 20]
        cmp %EAX, %ECX
        fxch %ST(1)
        fcmovae %ST(0), %ST(1)
***     fstp %ST(1)
        ret

Instead of:

foldSel3:
        mov %AL, BYTE PTR [%ESP + 4]
        fld DWORD PTR [%ESP + 8]
        fld DWORD PTR [%ESP + 12]
        mov %EAX, DWORD PTR [%ESP + 16]
        mov %ECX, DWORD PTR [%ESP + 20]
        cmp %EAX, %ECX
        fxch %ST(1)
        fcmovae %ST(0), %ST(1)
***     fxch %ST(1)
***     fstp %ST(0)
        ret

In practice, this only effects code size: performance should be basically
unaffected.

llvm-svn: 12588