* Add shift_left fix for signed integers by possibly negative values + regression test

* add required condition on shift_left integer fix

* add type check to shift_left minimum condition

* fix constant folding of shifts with |b| >= type.bits() for types that allow overflow (failes correctness/simplify test)

* make regression tests use scoped bindings

* change condition in case int24/int48 proposal happens soon

* revert changes based on overflow expectations

* add more regression tests

* clarify comment

* add shift_left min handler for b only UB

* fix clang-tidy complaint

* relax shift_left of non-negative value constraint

* pull case outside of unnecessary preconditions

* fix clang-format complaint

* fix broken precondition

* add typecheck to possibly save a can_prove() call

* add easy-out type check to precondition

* Add descriptive comment to bug fix + add another early-exit precondition

Co-authored-by: Steven Johnson <srj@google.com>