Separate Operand store logic from EmitStore()

Separate Operand store logic from EmitStore()

The new Store() helper function can store Operand instances independent
from SPIR-V instructions. This allows reuse of this logic for other
instructions that need to store to memory, like Modf and Frexp.

Bug: b/153641251
Change-Id: I453bb7cd24ba26b9a23d73568dc3374a52a36073
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43695
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>