[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.
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