[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

llvm-svn: 343999