[MLIR][SPIRV] Create new ctx for deserialization in roundtrips.

Roundtripping SPIR-V modules used the same MLIRContext object for both
ways of the trip. This resulted in deserialization using a context
object already containing Types constructed during serialization.
This commit rectifies that by creating a new MLIRContext during
deserialization.

Reviewed By: mravishankar, antiagainst

Differential Revision: https://reviews.llvm.org/D87692