lower "X = seteq Y, Z" to '(shr (ctlz (xor Y, Z)), 5)' instead of

'(shr (ctlz (sub Y, Z)), 5)'.

The use of xor better exposes the operation to bit-twiddling logic in the
dag combiner.  For example, this:

typedef struct {
  unsigned prefix : 4;
  unsigned code : 4;
  unsigned unsigned_p : 4;
} tree_common;

int foo(tree_common *a, tree_common *b) {
  return a->code == b->code;
}

Now compiles to:

_foo:
        lwz r2, 0(r4)
        lwz r3, 0(r3)
        xor r2, r3, r2
        rlwinm r2, r2, 28, 28, 31
        cntlzw r2, r2
        srwi r3, r2, 5
        blr

instead of:

_foo:
        lbz r2, 3(r4)
        lbz r3, 3(r3)
        srwi r2, r2, 4
        srwi r3, r3, 4
        subf r2, r2, r3
        cntlzw r2, r2
        srwi r3, r2, 5
        blr

saving a cycle.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31725 91177308-0d34-0410-b5e6-96231b3b80d8

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