SpirvRoutine holds reactor fields which must not outlive the scope of the reactor mutex.

While the routine was not directly referenced outside of ComputeProgram::generate(), the destructor of the SpirvRoutine's rr::Variables was touching reactor, leading to data races.

Bug: b/133127573
Change-Id: I854e079efdc0ef2d5b1ae67f014fe7148ad8f892
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31838
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>