[Thumb2] support `movs pc, lr` alias for `subs pc, lr, #0`/`eret`

This is used by the Linux kernel built with CONFIG_THUMB2_KERNEL.

Because different operands are not permitted to `movs`, the diagnostics now provide multiple suggestions along the lines of using a non-pc destination operand or lr source operand.

Forked from D95586.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D96304