Avoid a bitcast FFI call in transmuting

For things that only change the valid ranges, we can just skip the `LLVMBuildBitCast` call.

I tried to tweak this a bit more and broke stuff, so I also added some extra tests for that as we apparently didn't have coverage.

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -1123,7 +1123,7 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // While optimizations will remove no-op transmutes, they might still be
     // there in debug or things that aren't no-op in MIR because they change
     // the Rust type but not the underlying layout/niche.
-    if from_scalar == to_scalar && from_backend_ty == to_backend_ty {
+    if from_scalar == to_scalar {
         return imm;
     }
 
@@ -1142,7 +1142,13 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     assume_scalar_range(bx, imm, from_scalar, from_backend_ty);
 
     imm = match (from_scalar.primitive(), to_scalar.primitive()) {
-        (Int(..) | Float(_), Int(..) | Float(_)) => bx.bitcast(imm, to_backend_ty),
+        (Int(..) | Float(_), Int(..) | Float(_)) => {
+            if from_backend_ty == to_backend_ty {
+                imm
+            } else {
+                bx.bitcast(imm, to_backend_ty)
+            }
+        }
         (Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
         (Int(..), Pointer(..)) => bx.ptradd(bx.const_null(bx.type_ptr()), imm),
         (Pointer(..), Int(..)) => {