Rollup merge of #117832 - RalfJung:interpret-shift, r=cjgillot

interpret: simplify handling of shifts by no longer trying to handle signed and unsigned shift amounts in the same branch

While we're at it, also update comments in codegen and MIR building related to shifts, and fix the overflow error printed by Miri on negative shift amounts.

Author: compiler-errors

tests/fail/intrinsics/unchecked_shl2.rs

@@ -0,0 +1,9 @@
+#![feature(core_intrinsics)]
+use std::intrinsics;
+
+fn main() {
+    unsafe {
+        let _n = intrinsics::unchecked_shl(1i8, -1);
+        //~^ ERROR: overflowing shift by -1 in `unchecked_shl`
+    }
+}

tests/fail/intrinsics/unchecked_shl2.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: overflowing shift by -1 in `unchecked_shl`
+  --> $DIR/unchecked_shl2.rs:LL:CC
+   |
+LL |         let _n = intrinsics::unchecked_shl(1i8, -1);
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/unchecked_shl2.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+