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[AArch64][AsmParser] Remove 'x31' alias for 'sp/xzr' register.

Authored by CarolineConcatto on Oct 23 2020, 2:37 AM.

Description

[AArch64][AsmParser] Remove 'x31' alias for 'sp/xzr' register.

Only the aliases 'xzr' and 'sp' exist for the physical register x31.
The reason for wanting to remove the alias 'x31' is because it allows users
to write invalid asm that is not accepted by the GNU assembler.

Is there any objection to removing this alias? Or do we want to keep
this for compatibility with existing code that uses w31/x31?

Differential Revision: https://reviews.llvm.org/D90153