HLSL: Fix possibly incorrect type conversion in StoreN and LoadN

HLSL: Fix possibly incorrect type conversion in StoreN and LoadN

Set type to r-value resulting from indexing vector, to prevent
float->uint conversion when source is already uint. Resulting
OpConvertFToU would otherwise fail validation because source is
already uint.
For LoadN, incorrect uint->float->uint can be avoided; fixing
potential truncation of big integer values.