compiler, transform: remove runtime.isnil hack

This hack was originally introduced in
https://github.com/tinygo-org/tinygo/pull/251 to fix an escape analysis
regression after https://github.com/tinygo-org/tinygo/pull/222
introduced nil checks. Since a new optimization in LLVM (see
https://reviews.llvm.org/D60047) this hack is not necessary anymore and
can be removed.

I've compared all regular tests and smoke tests before and after to
check the size. In most cases this change was an improvement although
there are a few regressions.

transform/optimizer.go

@@ -98,24 +98,6 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		OptimizeAllocs(mod)
 		OptimizeStringToBytes(mod)
 
-		// Lower runtime.isnil calls to regular nil comparisons.
-		isnil := mod.NamedFunction("runtime.isnil")
-		if !isnil.IsNil() {
-			builder := mod.Context().NewBuilder()
-			defer builder.Dispose()
-			for _, use := range getUses(isnil) {
-				builder.SetInsertPointBefore(use)
-				ptr := use.Operand(0)
-				if !ptr.IsABitCastInst().IsNil() {
-					ptr = ptr.Operand(0)
-				}
-				nilptr := llvm.ConstPointerNull(ptr.Type())
-				icmp := builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
-				use.ReplaceAllUsesWith(icmp)
-				use.EraseFromParentAsInstruction()
-			}
-		}
-
 	} else {
 		// Must be run at any optimization level.
 		err := LowerInterfaces(mod)