cmd/compile: unnecessary copy in regalloc

[josharian]

After CLs 297049 and 297050, generate code for shrVU_g in math/big.

The result includes these two instructions:

v121 00026 (+181) LEAQ 1(R10), R8
v107 00027 (181) MOVQ R8, CX

After this, R8 is dead. So this could have just been one instruction, LEAQ 1(R10), CX. Here's the relevant block after regalloc:

This is the sort of thing a peephole pass could help with (#15300), but maybe now that Values have Uses counts there's a way to detect and improve this in regalloc directly.

cc @randall77 @cherrymui