Make BitMemoryRegion::LoadBit() ASAN friendly.

The __attribute__((no_sanitize_address)) is not respected
when the function is inlined. Work around that by reading
only the needed byte.

Test: m test-art-host-gtest
Test: aosp_taimen-userdebug boots.
Bug: 117183272
Change-Id: I4fba89b8981373f4dd041609a00ed3750e00e2c3