egraph opts: fix uextend-of-i32. (#5061)

This is a simple error in the const-prop rules: uextend was not
masking iconst's u64 immediate when extending from i32 to
i64. Arguably an iconst.i32 should not have nonzero bits in the upper
32 of its immediate, but that's a separate design question. For now,
if our invariant is that the upper bits are ignored, then it is
required to mask the bits when const-evaling a `uextend`.

Fixes #5047.

cranelift/codegen/src/isle_prelude.rs

@@ -95,6 +95,11 @@ macro_rules! isle_common_prelude_methods {
             Some(x as u32 as i32 as i64 as u64)
         }
 
+        #[inline]
+        fn u64_uextend_u32(&mut self, x: u64) -> Option<u64> {
+            Some(x & 0xffff_ffff)
+        }
+
         #[inline]
         fn ty_bits(&mut self, ty: Type) -> Option<u8> {
             use std::convert::TryInto;