Subzero: map optimization level none/less to m1

Opt_0 and Opt_1 are not implemented, so they weren't useful. With this
change, we can at least run Subzero's "0-1" optimization level to
compare against "02".

Bug: b/144688789
Change-Id: Ic6a5af572ed9a543f76bbc7074f87771f8da1980
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38476
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Reactor/SubzeroReactor.cpp

diff --git a/src/Reactor/SubzeroReactor.cpp b/src/Reactor/SubzeroReactor.cpp
index 1ec7404..93db311 100644
--- a/src/Reactor/SubzeroReactor.cpp
+++ b/src/Reactor/SubzeroReactor.cpp
@@ -98,8 +98,9 @@
 	{
 		switch (level)
 		{
-			case rr::Optimization::Level::None:       return Ice::Opt_0;
-			case rr::Optimization::Level::Less:       return Ice::Opt_1;
+			// Note that Opt_0 and Opt_1 are not implemented by Subzero
+			case rr::Optimization::Level::None:       return Ice::Opt_m1;
+			case rr::Optimization::Level::Less:       return Ice::Opt_m1;
 			case rr::Optimization::Level::Default:    return Ice::Opt_2;
 			case rr::Optimization::Level::Aggressive: return Ice::Opt_2;
 			default: UNREACHABLE("Unknown Optimization Level %d", int(level));