IRgen: Move EmitStoreThroughBitfieldLValue to use new...

IRgen: Move EmitStoreThroughBitfieldLValue to use new CGBitfieldInfo::AccessInfo decomposition, instead of computing the access policy itself.
 - Sadly, this doesn't seem to give any .ll size win so far. It is possible to make this routine significantly smarter & avoid various shifting, masking, and zext/sext, but I'm not really convinced it is worth it. It is tricky, and this is really instcombine's job.

 - No intended functionality change; the test case is just to increase coverage & serves as a demo file, it worked before this commit.

llvm-svn: 101222