constant fold FP_ROUND_INREG, ZERO_EXTEND_INREG, and SIGN_EXTEND_INREG

This allows the alpha backend to compile:

bool %test(uint %P) {
        %c = seteq uint %P, 0
        ret bool %c
}

into:

test:
        ldgp $29, 0($27)
        ZAP $16,240,$0
        CMPEQ $0,0,$0
        AND $0,1,$0
        ret $31,($26),1

instead of:

test:
        ldgp $29, 0($27)
        ZAP $16,240,$0
        ldiq $1,0
        ZAP $1,240,$1
        CMPEQ $0,$1,$0
        AND $0,1,$0
        ret $31,($26),1

... and fixes PR534.

llvm-svn: 20534