AST: Fix shift conversions, which don't require matching types.

The base and shift amount need to be integers, but not of the same type.
This fixes #1296 and replaces #1297.

Test/spv.int64.frag

@@ -116,6 +116,8 @@ void operators()
     i64  = i64 % i;
 
     // Shift
+    u64v = u64v << i;
+    i64 = i64 >> uv.y;
     u64v <<= i;
     i64  >>= uv.y;