codegen: take gc roots (and alloca alignment) more seriously
Due to limitations in the LLVM implementation, we are forced to emit
fairly bad code here. But we need to make sure it is still correct with
respect to GC rooting.
The PR 50833c84d454ef989797e035294ba27b3cca79b7 also changed the meaning
of haspadding without changing all of the existing users to use the new
equivalent flag (haspadding || !isbitsegal), incurring additional
breakage here as well and needing more tests for that.
Fixes #54720
