The LLVMContext object is used for caching LLVM types, eliminating
duplicate constants, and things like handling diagnostics information.
Previously we created one for the JITBuilder as well as the JITRoutine.
While having multiple ones is mostly benign from a correctness point of
view, it adds unnecessary overhead.

This change reuses the context created for the JITBuilder object (which
is used during IR construction), for the JITRoutine (which uses it for
actual compilation).

Bug: b/177024837
Change-Id: Iefbbd7fbeb53e67ab41914fad471dd94d9755311
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51568
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>