pvanhout b3b3cb2d2f [AMDGPU] Less aggressively break large PHIs ...

In some cases, breaking large PHIs can very negatively affect
performance (3x more instructions observed in a particular test case).

This patch adds some basic profitability heuristics to help with some of these issues without affecting the "good" cases.
e.g. avoid breaking PHIs if it causes back-and-forth between vector/scalar form for no good reason.

Fixes SWDEV-392803
Fixes SWDEV-393781
Fixes SWDEV-394228

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D147786

2023-04-14 15:41:26 +02:00

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