[RISCV] Lower inline asm m with offset to register+imm.

As part of D145584, we noticed that llvm was generating suboptimal code
for constraint m when the operand can be be lowered to reg+imm form: it
was being selected as a single register rather than register+imm. This
caused an unnecessary 'addi' to be gen for each m constraint.

This patch changes llvm to select register+imm. This might generate code
that cannot be assembled, but matches gcc's behavior.

Reviewed By: craig.topper, kito-cheng

Differential Revision: https://reviews.llvm.org/D146245