Bitfields generate bad code when casted

[llvm-beanz]

Description
HLSL "Flat" casting bitfields does not properly mask bitfield members.

Steps to Reproduce
https://godbolt.org/z/MEKdTYxWM

Actual Behavior
Bit field members are read as full storage, not correctly masked, so instead of returning 3, this code returns 101057538 (0x6060402).

Environment

• DXC version: main
• Host Operating System: all

[s-perron]

If you are going to change this behaviour, you will need to update the SPIR-V backend to match it. We simply matched the DXIl behaviour when implementing. Nobody reported a bug related to bitfields, but we epxect that people will rely on unspecified behvaiour.

See #4906 for the issue rleated to flat casting in spir-v.

EDIT: Actually the spir-v code in this case is wrong in many different ways: https://godbolt.org/z/vcjGYM673.