When we have a two-address instruction where the input cannot be clobbered

and is already available, instead of falling back to emitting a load, fall
back to emitting a reg-reg copy.  This generates significantly better code
for some SSE testcases, as SSE has lots of two-address instructions and
none of them are read/modify/write.  As one example, this change does:

        pshufd %XMM5, XMMWORD PTR [%ESP + 84], 255
        xorps %XMM2, %XMM5
        cmpltps %XMM1, %XMM0
-       movaps XMMWORD PTR [%ESP + 52], %XMM0
-       movapd %XMM6, XMMWORD PTR [%ESP + 52]
+       movaps %XMM6, %XMM0
        cmpltps %XMM6, XMMWORD PTR [%ESP + 68]
        movapd XMMWORD PTR [%ESP + 52], %XMM6
        movaps %XMM6, %XMM0
        cmpltps %XMM6, XMMWORD PTR [%ESP + 36]
        cmpltps %XMM3, %XMM0
-       movaps XMMWORD PTR [%ESP + 20], %XMM0
-       movapd %XMM7, XMMWORD PTR [%ESP + 20]
+       movaps %XMM7, %XMM0
        cmpltps %XMM7, XMMWORD PTR [%ESP + 4]
        movapd XMMWORD PTR [%ESP + 20], %XMM7
        cmpltps %XMM4, %XMM0

... which is far better than a store followed by a load!

llvm-svn: 28001