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[DAGCombine] Add hook to allow target specific test for sqrt input

Authored by steven.zhang on Nov 24 2020, 9:37 PM.

Description

[DAGCombine] Add hook to allow target specific test for sqrt input

PowerPC has instruction ftsqrt/xstsqrtdp etc to do the input test for software square root.
LLVM now tests it with smallest normalized value using abs + setcc. We should add hook to
target that has test instructions.

Reviewed By: Spatel, Chen Zheng, Qiu Chao Fang

Differential Revision: https://reviews.llvm.org/D80706