all: use the new LLVM pass manager

The old LLVM pass manager is deprecated and should not be used anymore.
Moreover, the pass manager builder (which we used to set up a pass
pipeline) is actually removed from LLVM entirely in LLVM 17:
https://reviews.llvm.org/D145387
https://reviews.llvm.org/D145835

The new pass manager does change the binary size in many cases: both
growing and shrinking it. However, on average the binary size remains
more or less the same.

This is needed as a preparation for LLVM 17.

compiler/compiler_test.go

@@ -91,14 +91,12 @@ func TestCompiler(t *testing.T) {
 			}
 
 			// Optimize IR a little.
-			funcPasses := llvm.NewFunctionPassManagerForModule(mod)
-			defer funcPasses.Dispose()
-			funcPasses.AddInstructionCombiningPass()
-			funcPasses.InitializeFunc()
-			for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
-				funcPasses.RunFunc(fn)
+			passOptions := llvm.NewPassBuilderOptions()
+			defer passOptions.Dispose()
+			err = mod.RunPasses("instcombine", llvm.TargetMachine{}, passOptions)
+			if err != nil {
+				t.Error(err)
 			}
-			funcPasses.FinalizeFunc()
 
 			outFilePrefix := tc.file[:len(tc.file)-3]
 			if tc.target != "" {