llvm-project

bae8f133 - Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (#134408)

Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (#134408) This tries to reland #123632 (previously reverted by commit 6b1db79887df19bc8e8c946108966aa6021c8b87) This PR aims to fix coalescing of SUBREG_TO_REG when sub-register liveness tracking is enabled and this is now the so-manieth reincarnation of this effort :) This change is needed in order to enable subreg liveness tracking for AArch64, because without the implicit-def, Machine Copy Propagation would remove a 'redundant' copy because it doesn't realise that the top 32-bits of the register are zeroed, which subsequent instructions rely on. Changes compared to previous PR: * Rather than updating all instructions that define the source register (SrcReg) of the SUBREG_TO_REG, this new approach only updates instructions that define SrcReg when they dominate the SUBREG_TO_REG. The live-ranges are updated accordingly.