Auto merge of #112307 - lcnr:operand-ref, r=compiler-errors

mir opt + codegen: handle subtyping

fixes #107205

the same issue was caused in multiple places:
- mir opts: both copy and destination propagation
- codegen: assigning operands to locals (which also propagates values)

I changed codegen to always update the type in the operands used for locals which should guard against any new occurrences of this bug going forward. I don't know how to make mir optimizations more resilient here. Hopefully the added tests will be enough to detect any trivially wrong optimizations going forward.

Author: bors