Put LLVM IR verification behind a default-disabled flag

Verification can take upwards of 10% of a dEQP test's execution time.

Verification is enabled for Kokoro builds and unit tests as silently regressing would be a Bad Thing.

Verification is also enabled for debug (!NDEBUG) builds.

Bug: b/135609394
Change-Id: I6c354c40f524e1e8f3e08f24129b7e008163b4bd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33215
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/Reactor/LLVMReactor.cpp

diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index d19cf52..a65f843 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -1051,14 +1051,13 @@
 			::module->print(file, 0);
 		}
 
-		// FIXME: Disable for release builds once heavy development is over.
-		bool verifyIR = true;
-		if(verifyIR)
+#if defined(ENABLE_RR_LLVM_IR_VERIFICATION) || !defined(NDEBUG)
 		{
 			llvm::legacy::PassManager pm;
 			pm.add(llvm::createVerifierPass());
 			pm.run(*::module);
 		}
+#endif // defined(ENABLE_RR_LLVM_IR_VERIFICATION) || !defined(NDEBUG)
 
 		if(runOptimizations)
 		{