Remove support for ZERO_EXTEND_INREG. This pessimizes code, genering stuff

like this:

        ldah $1,1($31)
        lda $1,-1($1)
        and $0,$1,$24

instead of this:

        zap $0,252,$24

To get this back, the selector should recognize the ISD::AND case where this
happens and emit the appropriate ZAP instruction.

llvm-svn: 21270