Teach the dag combiner to turn a truncate/sign_extend pair into a sextinreg

Teach the dag combiner to turn a truncate/sign_extend pair into a sextinreg
when the types match up.  This allows the X86 backend to compile:

sbyte %toggle_value(sbyte* %tmp.1) {
        %tmp.2 = load sbyte* %tmp.1
        ret sbyte %tmp.2
}

to this:

_toggle_value:
        mov %EAX, DWORD PTR [%ESP + 4]
        movsx %EAX, BYTE PTR [%EAX]
        ret

instead of this:

_toggle_value:
        mov %EAX, DWORD PTR [%ESP + 4]
        movsx %EAX, BYTE PTR [%EAX]
        movsx %EAX, %AL
        ret

noticed in Shootout/objinst.

-Chris

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24630 91177308-0d34-0410-b5e6-96231b3b80d8