The LLVM MemorySanitizer pass was added for -fsanitizer=memory builds
when REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION is defined, but
LLVMJIT.cpp includes LLVMReactor.hpp, which in turn includes Reactor.hpp
which defines REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION to 0 if
not previously defined, which will always enable it.

Use #if instead of #ifdef to conditionally enable instrumentation based
on REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION's value.

Bug: b/155148722
Change-Id: I337832d942fcc576afac4d0c1d89c9fa42974cbe
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50288
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>