tint/reader/spirv: Generate 'i' suffixed literals

For all i32 literal values.

Reduces risk of the SPIR-V reader producing WGSL that behaves
differently, when abstract-integers are fully implemented.

Bug: tint:1504
Change-Id: Ieaf8afec5b09c7978c75a38c6ed144633ddc017e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/88843
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

test/tint/ptr_ref/store/local/struct_field.spvasm.expected.wgsl

@@ -4,7 +4,7 @@ struct S {
 
 fn main_1() {
   var V : S;
-  V.i = 5;
+  V.i = 5i;
   return;
 }