Do not scan relocations twice for MIPS.

Previously, relocations for MIPS were scanned twice; once in regular
scanRelocs() and the other is in scanRelocsForThunks. In the former
function, we computed types of relocations and skipped R_THUNK relocations.
In the latter function, we computed the same value again and skipped
all but R_THUNK relocations. It was wasteful. This patch fixes that.
Now R_THUNK relocations are handled in the regular manner.

llvm-svn: 273346