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/filetests/filetests/egraph/algebraic.clif

@@ -3,7 +3,7 @@ set opt_level=none
 set use_egraphs=true
 target x86_64
 
-function %f(i32) -> i32 {
+function %f0(i32) -> i32 {
 block0(v0: i32):
     v1 = iconst.i32 2
     v2 = imul v0, v1
@@ -11,3 +11,12 @@ block0(v0: i32):
     ; nextln: return v1
     return v2
 }
+
+function %f1() -> i64 {
+block0:
+  v0 = iconst.i32 0xffff_ffff_9876_5432
+  v1 = uextend.i64 v0
+  return v1
+  ; check: v0 = iconst.i64 0x9876_5432
+  ; nextln: return v0  ; v0 = 0x9876_5432
+}