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/prelude.isle

@@ -125,6 +125,9 @@
 (decl pure u64_sextend_u32 (u64) u64)
 (extern constructor u64_sextend_u32 u64_sextend_u32)
 
+(decl pure u64_uextend_u32 (u64) u64)
+(extern constructor u64_uextend_u32 u64_uextend_u32)
+
 (decl u64_is_zero (bool) u64)
 (extern extractor infallible u64_is_zero u64_is_zero)