[PATCH] [TableGen] Add alternative register matching support to AsmMatcher

[Dylan McKay]

This commit adds a new flag ShouldEmitMatchRegisterAltName to
include/Target/Target.td, and generates an alternative register name
matching function named MatchRegisterAlias to the generated AsmMatcher if
it is enabled.

Backends can use this to parse alternative register names based on values
stored
in TableGen files. The AVR-LLVM <https://github.com/avr-llvm/llvm> project
is using this function to parse special pointer registers, identified by X,
Y, and Z, which are alternative names to the GPR pairs r27:r26, r29:r28,
and r31:r30.

Files changed:

   - include/Target/Target.td
   - utils/TableGen/AsmMatcherEmitter.cpp

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