Use cute tblgen tricks to make zap handling more powerful. Specifically,

when the dag combiner simplifies an and mask, notice this and allow those bits
to be missing from the zap mask.

This compiles Alpha/zapnot4.ll into:

        sll $16,3,$0
        zapnot $0,3,$0
        ret $31,($26),1

instead of:

        ldah $0,1($31)
        lda $0,-8($0)
        sll $16,3,$1
        and $1,$0,$0
        ret $31,($26),1

It would be *really* nice to replace the hunk of code in the
AlphaISelDAGToDAG.cpp file that matches (and (srl (x, C), c2) into
(SRL (ZAPNOTi)) with a similar pattern, but I've spent enough time poking
at alpha.  Make andrew will do this.

llvm-svn: 30875

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