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[llvm][AArch64] Allow TB(N)Z to drop signext for sign bit tests.

Authored by fpetrogalli on Nov 9 2020, 10:18 AM.

Description

[llvm][AArch64] Allow TB(N)Z to drop signext for sign bit tests.

For example if the sign extension is only used in for TBZ, and the value is used elsewhere with a zero extension, this can eliminate a sign extension.

Reviewed By: samparker

Differential Revision: https://reviews.llvm.org/D90606

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