Poison compatibility is provided by emitting a freeze instruction, post load,
for all Lvalue bit-field store operations. This solution is indiscriminate to
the initialization state and thus the freeze instruction may hide poison during
subsequent re-assignments.
This patch implements the first proposed solution from the following RFC:
https://discourse.llvm.org/t/rfc-making-bit-field-codegen-poison-compatible/63250
The test-suite is passing; my current setup is not idea for timing information.