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[GlobalISel] Add a combine for ashr(shl x, c), c --> sext_inreg x, c'

Authored by aemerson on Aug 14 2020, 1:58 AM.

Description

[GlobalISel] Add a combine for ashr(shl x, c), c --> sext_inreg x, c'

By detecting this sign extend pattern early, we can uncover opportunities for
more optimizations.

Differential Revision: https://reviews.llvm.org/D85965

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