[MemProf][NFC] Rename DefaultShadowGranularity to DefaultMemGranulari… (#79412)

…ty in instrumentation code, be consistent with runtime

In runtime code, the size of memory block mapped to a single shadow
location is called MEM_GRANULARITY.
In instrumentation code, the size of memory block mapped to a single
shadow location is called DefaultShadowGranularity.
Actually, the SHADOW_GRANULARITY is 8 (1 << SHADOW_SCALE), and the
MEM_GRANULARITY is 64.
The wording of DefaultShadowGranularity in instrumentation code is a bit
misleading, this patch renames DefaultShadowGranularity to
DefaultMemGranularity, be consistent with runtime.