Revert "[InferAttrs] Add writeonly to all the math functions"

This reverts commit ea75be3 and
1eb5b6e.

That commit caused crashes with compilation e.g. like this
(not fixed by the follow-up commit):

$ cat sqrt.c
float a;
b() { sqrt(a); }
$ clang -target x86_64-linux-gnu -c -O2 sqrt.c
Attributes 'readnone and writeonly' are incompatible!
  %sqrtf = tail call float @sqrtf(float %0) #1
in function b
fatal error: error in backend: Broken function found, compilation aborted!

llvm/lib/Transforms/Utils/BuildLibCalls.cpp

@@ -34,7 +34,6 @@ STATISTIC(NumReadNone, "Number of functions inferred as readnone");
 STATISTIC(NumInaccessibleMemOnly,
           "Number of functions inferred as inaccessiblememonly");
 STATISTIC(NumReadOnly, "Number of functions inferred as readonly");
-STATISTIC(NumWriteOnly, "Number of functions inferred as writeonly");
 STATISTIC(NumArgMemOnly, "Number of functions inferred as argmemonly");
 STATISTIC(NumInaccessibleMemOrArgMemOnly,
           "Number of functions inferred as inaccessiblemem_or_argmemonly");
@@ -72,19 +71,6 @@ static bool setOnlyReadsMemory(Function &F) {
   return true;
 }
 
-static bool setDoesNotReadMemory(Function &F) {
-  if (F.doesNotReadMemory()) // writeonly or readnone
-    return false;
-  ++NumWriteOnly;
-  if (F.hasFnAttribute(Attribute::ReadOnly)) {
-    F.removeFnAttr(Attribute::ReadOnly);
-    F.setDoesNotAccessMemory();
-  } else {
-    F.setDoesNotReadMemory();
-  }
-  return true;
-}
-
 static bool setOnlyAccessesArgMemory(Function &F) {
   if (F.onlyAccessesArgMemory())
     return false;
@@ -1185,7 +1171,6 @@ bool llvm::inferLibFuncAttributes(Function &F, const TargetLibraryInfo &TLI) {
   case LibFunc_truncl:
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotFreeMemory(F);
-    Changed |= setDoesNotReadMemory(F);
     Changed |= setWillReturn(F);
     return Changed;
   default: