Fix a FIXME: Select instructions on longs were miscompiled.

While we're at it, improve codegen of select instructions.  For this
testcase:

int %test(bool %C, int %A, int %B) {
  %D = select bool %C, int %A, int %B
  ret int %D
}

We used to generate this code:

_test:
        cmpwi cr0, r3, 0
        bne .LBB_test_2 ;
.LBB_test_1:    ;
        b .LBB_test_3   ;
.LBB_test_2:    ;
        or r5, r4, r4
.LBB_test_3:    ;
        or r3, r5, r5
        blr

Now we emit:

_test:
        cmpwi cr0, r3, 0
        bne .LBB_test_2 ;
.LBB_test_1:    ;
        or r4, r5, r5
.LBB_test_2:    ;
        or r3, r4, r4
        blr

-Chris

llvm-svn: 19214