Simon Pilgrim 0cf48a7065 [InstCombine] visitTrunc - trunc (*shr (trunc A), C) --> trunc(*shr A, C) ...

Attempt to fold trunc (*shr (trunc A), C) --> trunc(*shr A, C) iff the shift amount if small enough that all zero/sign bits created by the shift are removed by the last trunc.

Helps fix the regressions encountered in D88316.

I've tweaked a couple of shift values as suggested by @lebedev.ri to ensure we have coverage of shift values close (above/below) to the max limit.

Differential Revision: https://reviews.llvm.org/D88429

2020-09-29 18:27:42 +01:00

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