[DebugInfo] Fix debug information label tests
Remove the space in the asm check so that the expression is more general
and can also capture MIPS labels which can be surrounded by braces, e.g.:
.4byte ($tmp1) # DW_AT_low_pc
Also change optimization level to O0 because the DW_TAG_label does not
appear on MIPS when -O2 is used.
Patch by Milos Stojanovic.
Differential Revision: https://reviews.llvm.org/D52901