Minor tweak to the branch selector. When emitting a two-way branch, and if

we're in a single-mbb loop, make sure to emit the backwards branch as the
conditional branch instead of the uncond branch.  For example, emit this:

LBBl29_z__44:
        stw r9, 0(r15)
        stw r9, 4(r15)
        stw r9, 8(r15)
        stw r9, 12(r15)
        addi r15, r15, 16
        addi r8, r8, 1
        cmpw cr0, r8, r28
        ble cr0, LBBl29_z__44
        b LBBl29_z__48                   *** NOT PART OF LOOP

Instead of:

LBBl29_z__44:
        stw r9, 0(r15)
        stw r9, 4(r15)
        stw r9, 8(r15)
        stw r9, 12(r15)
        addi r15, r15, 16
        addi r8, r8, 1
        cmpw cr0, r8, r28
        bgt cr0, LBBl29_z__48            *** PART OF LOOP!
        b LBBl29_z__44

The former sequence has one fewer dispatch group for the loop body.

llvm-svn: 23582