[RISCV] Correct even register check for amocas.

We were checking that the encoding within our internal list of
registers was even. This worked today because X0 happens to have
an even value in that enum. This can break if any registers are
added before X0.

The correct check is to make sure it has an even offset from X0.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D155104