Implement full support for parsing primary expressions. We can now parse

all of health and voronoi (ignoring directives).  We only get 409 lines into
176.gcc though because we don't have binary operators yet:

Parsing 176.gcc.llc.s:409: unexpected token in operand list
	movsbl	_arityvec+1(,%edi,8), %eax
	      	         ^

llvm-svn: 73877