[MC] Fixed parsing of macro arguments where expressions with spaces are present.

Summary:
Fixed an issue for mips with an instruction such as 'sdc1 $f1, 272 +8(a0)' which has a space between '272' and '+'. The parser would then parse '272' and '+8' as two arguments instead of a single expression resulting in one too many arguments in the pseudo instruction.
The reason that the test case has been changed is so that the expected
output matches the output of the GNU assembler.

Reviewers: vkalintiris, dsanders

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D13592

llvm-svn: 260521