aco/ra: fix get_reg_for_operand() in case of stride mismatches

We have to clear the register file from the previous operand as otherwise, there might be no space left.

Totals from 5 (0.00% of 134572) affected shaders: (GFX10.3)
CodeSize: 21144 -> 21000 (-0.68%); split: -0.72%, +0.04%
Instrs: 3738 -> 3720 (-0.48%); split: -0.51%, +0.03%
Latency: 517229 -> 516319 (-0.18%); split: -0.18%, +0.00%
InvThroughput: 49068 -> 48902 (-0.34%); split: -0.38%, +0.04%
Copies: 501 -> 483 (-3.59%); split: -3.79%, +0.20%

Cc: mesa-stable

Found when testing !13576 (merged)