Implement Regression/CodeGen/X86/rotate.ll: emit rotate instructions (which

typically cost 1 cycle) instead of shld/shrd instruction (which are typically
6 or more cycles).  This also saves code space.

For example, instead of emitting:

rotr:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %CL, BYTE PTR [%ESP + 8]
        shrd %EAX, %EAX, %CL
        ret
rotli:
        mov %EAX, DWORD PTR [%ESP + 4]
        shrd %EAX, %EAX, 27
        ret

Emit:

rotr32:
        mov %CL, BYTE PTR [%ESP + 8]
        mov %EAX, DWORD PTR [%ESP + 4]
        ror %EAX, %CL
        ret
rotli32:
        mov %EAX, DWORD PTR [%ESP + 4]
        ror %EAX, 27
        ret

We also emit byte rotate instructions which do not have a sh[lr]d counterpart
at all.

llvm-svn: 19692