Fix a bug which caused us to never be able to use signed comparisons for

equality comparisons of a constant.  This allows us to codegen the 'sintzero'
loop in PR1288 as:

LBB1_1: ;cond_next
        li r4, 0
        addi r2, r2, 1
        stw r4, 0(r3)
        addi r3, r3, 4
        cmpwi cr0, r2, -1
        bne cr0, LBB1_1 ;cond_next

instead of:

LBB1_1: ;cond_next
        addi r2, r2, 1
        li r4, 0
        xoris r5, r2, 65535
        stw r4, 0(r3)
        addi r3, r3, 4
        cmplwi cr0, r5, 65535
        bne cr0, LBB1_1 ;cond_next

This implements CodeGen/PowerPC/compare-simm.ll, and also cuts 74
instructions out of kc++.

llvm-svn: 35590