When lowering SELECT_CC, see if the input is a lowered SETCC. If so, fold

When lowering SELECT_CC, see if the input is a lowered SETCC.  If so, fold
the two operations together.  This allows us to compile this:

void %two(int %a, int* %b) {
        %tmp.2 = seteq int %a, 0
        %tmp.0.0 = select bool %tmp.2, int 10, int 20
        store int %tmp.0.0, int* %b
        ret void
}

into:

two:
        save -96, %o6, %o6
        or %g0, 20, %l0
        or %g0, 10, %l1
        subcc %i0, 0, %l2
        be .LBBtwo_2    ! entry
        nop
.LBBtwo_1:      ! entry
        or %g0, %l0, %l1
.LBBtwo_2:      ! entry
        st %l1, [%i1]
        restore %g0, %g0, %g0
        retl
        nop

instead of:

two:
        save -96, %o6, %o6
        sethi 0, %l0
        or %g0, 1, %l1
        or %g0, 20, %l2
        or %g0, 10, %l3
        subcc %i0, 0, %l4
        be .LBBtwo_2    ! entry
        nop
.LBBtwo_1:      ! entry
        or %g0, %l0, %l1
.LBBtwo_2:      ! entry
        subcc %l1, 0, %l0
        bne .LBBtwo_4   ! entry
        nop
.LBBtwo_3:      ! entry
        or %g0, %l2, %l3
.LBBtwo_4:      ! entry
        st %l3, [%i1]
        restore %g0, %g0, %g0
        retl
        nop

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25806 91177308-0d34-0410-b5e6-96231b3b80d8