Generate separate stores for partially swizzled memory stores

Generate separate stores for partially swizzled memory stores

Full vector and fully specified vector swizzle stores are not affected by this change, only partial swizzles ie swizzles with fewer components than the vector being stored to.

Previously the vector being stored to loaded and any components not specified in the swizzle were used to create a full store to the vector.

While this change generates more SPIR-V instructions, it is necessary for correctness.

Fixes #2518.