[RegisterCoalescer] Fix subrange update when rematerialization widens a def (#151974)

Currently, when an instruction rematerialized by the register coalescer
defines more subregs of the destination register
than the original COPY instruction did, we only add dead defs for the
newly defined subregs if they were not defined anywhere
else. For example, consider something like this before
rematerialization:
```
 %0:reg64 = CONSTANT 1
 %1:reg128.sub_lo64_lo32 = COPY %0.lo32
 %1:reg128.sub_lo64_hi32 = ...
 ...
```
that would look like this after rematerializing `%0`:
```
 %0:reg64 = CONSTANT 2
 %1:reg128.sub_lo64 = CONSTANT 2
 %1:reg128.sub_lo64_hi32 = ...
 ...
```
A dead def would not be added for `%1.sub_lo64_hi32` at the 2nd
instruction because it's subrange wasn't empty beforehand.

0 files changed, 0 insertions, 0 deletions