[optnone] Make the optnone attribute effective at suppressing function

attribute and function argument attribute synthesizing and propagating.

As with the other uses of this attribute, the goal remains a best-effort
(no guarantees) attempt to not optimize the function or assume things
about the function when optimizing. This is particularly useful for
compiler testing, bisecting miscompiles, triaging things, etc. I was
hitting specific issues using optnone to isolate test code from a test
driver for my fuzz testing, and this is one step of fixing that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215538 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/FunctionAttrs/optnone.ll

@@ -0,0 +1,24 @@
+; RUN: opt < %s -functionattrs -S | FileCheck %s
+
+@x = global i32 0
+
+define void @test_opt(i8* %p) {
+; CHECK-LABEL: @test_opt
+; CHECK: (i8* nocapture readnone %p) #0 {
+  ret void
+}
+
+define void @test_optnone(i8* %p) noinline optnone {
+; CHECK-LABEL: @test_optnone
+; CHECK: (i8* %p) #1 {
+  ret void
+}
+
+declare i8 @strlen(i8*) noinline optnone
+; CHECK-LABEL: @strlen
+; CHECK: (i8*) #1
+
+; CHECK-LABEL: attributes #0
+; CHECK: = { readnone }
+; CHECK-LABEL: attributes #1
+; CHECK: = { noinline optnone }