[ARM, Asm] Use correct source location for register tokens

tryParseRegister advances the lexer, so we need to take copies of the start and
end locations of the register operand before calling it.

Previously, the caret in the diagnostic pointer to the comma after the r0
operand in the test, rather than the start of the operand.

Differential revision: https://reviews.llvm.org/D31537

llvm-svn: 314799