Optimizing: Rename `LoopInformation::GetBlocks()`...

... to `GetBlockMask()` to free up the name `GetBlocks()`
for returning an iteration range for actual blocks.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Flag: EXEMPT refactor
Change-Id: Ia7a75af26d5911d4ea4460604e1e46158859eda5