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[InstCombine] Transform 1.0/sqrt(X) * X to X/sqrt(X)

Authored by venkataramanan.kumar.llvm on Sep 2 2020, 5:23 AM.

Description

[InstCombine] Transform 1.0/sqrt(X) * X to X/sqrt(X)

These transforms will now be performed irrespective of the number of uses for the expression "1.0/sqrt(X)":
1.0/sqrt(X) * X => X/sqrt(X)
X * 1.0/sqrt(X) => X/sqrt(X)

We already handle more general cases, and we are intentionally not creating extra (and likely expensive)
fdiv ops in IR. This pattern is the exception to the rule because we always expect the Backend to reduce
X/sqrt(X) to sqrt(X), if it has the necessary (reassoc) fast-math-flags.

Ref: DagCombiner optimizes the X/sqrt(X) to sqrt(X).

Differential Revision: https://reviews.llvm.org/D86726