aarch64: actually copy first operand in convert_bfc_to_bfm()

aarch64: actually copy first operand in convert_bfc_to_bfm()

Commit 93562a343c ("[AArch64] PR target/20666, fix wrong encoding of
new introduced BFC pseudo") changed the destination operand to 0,
making the whole function invocation a no-op. We really want to copy
operand 0 (a register) to operand 1 (an immediate before coming here),
even if right now this likely is only a latent bug.