Emit trunc nuw for unchecked shifts and to_immediate_scalar

- For shifts this shrinks the IR by no longer needing an `assume` while still providing the UB information
- Having this on the `i8`→`i1` truncations will hopefully help with some places that have to load `i8`s or pass those in LLVM structs without range information

compiler/rustc_codegen_ssa/src/base.rs

@@ -24,7 +24,7 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::layout::{HasTyCtxt, HasTypingEnv, LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_session::Session;
-use rustc_session::config::{self, CrateType, EntryFnType, OptLevel, OutputType};
+use rustc_session::config::{self, CrateType, EntryFnType, OutputType};
 use rustc_span::{DUMMY_SP, Symbol, sym};
 use rustc_trait_selection::infer::{BoundRegionConversionTime, TyCtxtInferExt};
 use rustc_trait_selection::traits::{ObligationCause, ObligationCtxt};
@@ -364,13 +364,7 @@ pub(crate) fn build_shift_expr_rhs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let rhs_sz = bx.cx().int_width(rhs_llty);
     let lhs_sz = bx.cx().int_width(lhs_llty);
     if lhs_sz < rhs_sz {
-        if is_unchecked && bx.sess().opts.optimize != OptLevel::No {
-            // FIXME: Use `trunc nuw` once that's available
-            let inrange = bx.icmp(IntPredicate::IntULE, rhs, mask);
-            bx.assume(inrange);
-        }
-
-        bx.trunc(rhs, lhs_llty)
+        if is_unchecked { bx.unchecked_utrunc(rhs, lhs_llty) } else { bx.trunc(rhs, lhs_llty) }
     } else if lhs_sz > rhs_sz {
         // We zero-extend even if the RHS is signed. So e.g. `(x: i32) << -1i8` will zero-extend the
         // RHS to `255i32`. But then we mask the shift amount to be within the size of the LHS

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -340,6 +340,17 @@ pub trait BuilderMethods<'a, 'tcx>:
     }
 
     fn trunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
+    /// Produces the same value as [`Self::trunc`] (and defaults to that),
+    /// but is UB unless the *zero*-extending the result can reproduce `val`.
+    fn unchecked_utrunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value {
+        self.trunc(val, dest_ty)
+    }
+    /// Produces the same value as [`Self::trunc`] (and defaults to that),
+    /// but is UB unless the *sign*-extending the result can reproduce `val`.
+    fn unchecked_strunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value {
+        self.trunc(val, dest_ty)
+    }
+
     fn sext(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
     fn fptoui_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
     fn fptosi_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;