Store booleans as i8 in memory to improve optimizations by LLVM

LLVM doesn't really like types with a bit-width that isn't a multiple of
8 and disable various optimizations if it encounters such types used
with loads/stores. OTOH, booleans must be represented as i1 when used as
SSA values. To get the best results, we must use i1 for SSA values, and
i8 when storing the value to memory.

By using range asserts on loads, LLVM can eliminate the required
zero-extend and truncate operations.

Fixes #15203