[PATCH] [ARM64] Fix the misleading diagnostic on bad extend amount of reg+reg addressing mode.

[Tim Northover]

Hi Kevin,

I don't think that degree of hackery is justified for a slightly better diagnostic. ARM64 already suffers from too much C++ code in the AsmParser, We want to work towards decreasing that rather than making it worse.

Cheers.

Tim.

http://reviews.llvm.org/D3662