Rollup merge of #125137 - RalfJung:mir-sh, r=scottmcm

MIR operators: clarify Shl/Shr handling of negative offsets

"made unsigned" was not fully clear (made unsigned how? by using `abs`? no), so let's say "re-interpreted as an unsigned value of the same size" instead.

r? `@scottmcm`

compiler/rustc_middle/src/mir/syntax.rs

@@ -1480,13 +1480,17 @@ pub enum BinOp {
  BitOr,
  /// The `<<` operator (shift left)
  ///
- /// The offset is truncated to the size of the first operand and made unsigned before shifting.
+ /// The offset is (uniquely) determined as follows:
+ /// - it is "equal modulo LHS::BITS" to the RHS
+ /// - it is in the range `0..LHS::BITS`
  Shl,
  /// Like `Shl`, but is UB if the RHS >= LHS::BITS or RHS < 0
  ShlUnchecked,
  /// The `>>` operator (shift right)
  ///
- /// The offset is truncated to the size of the first operand and made unsigned before shifting.
+ /// The offset is (uniquely) determined as follows:
+ /// - it is "equal modulo LHS::BITS" to the RHS
+ /// - it is in the range `0..LHS::BITS`
  ///
  /// This is an arithmetic shift if the LHS is signed
  /// and a logical shift if the LHS is unsigned.