Rollup merge of #119077 - tmiasko:lint, r=cjgillot

Separate MIR lints from validation

Add a MIR lint pass, enabled with -Zlint-mir, which identifies undefined or
likely erroneous behaviour.

The initial implementation mostly migrates existing checks of this nature from
MIR validator, where they did not belong (those checks have false positives and
there is nothing inherently invalid about MIR with undefined behaviour).

Fixes #104736
Fixes #104843
Fixes #116079
Fixes #116736
Fixes #118990

Author: compiler-errors