egraph opts: fix uextend-of-i32.

[cfallin]

trafficstars

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.

[cfallin]

cc @afonso360

[bjorn3]

Arguably an iconst.i32 should not have nonzero bits in the upper 32 of its immediate

Iconst uses the signed Imm64 type, so it is not weird to see a sign extended iconst.