aco/optimizer_postRA: Distinguish overwritten untrackable and subdword.

aco/optimizer_postRA: Distinguish overwritten untrackable and subdword.

This allows is_overwritten_since to return false when the last
writer instruction of a register can't be tracked but we know it wasn't
written in the current block.

Fossil DB stats on Rembrandt (RDNA2):
Totals from 1163 (0.86% of 134906) affected shaders:
CodeSize: 9815920 -> 9805016 (-0.11%)
Instrs: 1843688 -> 1840962 (-0.15%)
Latency: 19219153 -> 19209171 (-0.05%)
InvThroughput: 3354375 -> 3353852 (-0.02%)

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16161>