rename "slow-unaligned-mem-under-32" to slow-unaligned-mem-16" (NFCI)

This is a follow-on suggested by:
http://reviews.llvm.org/D12154 ( http://reviews.llvm.org/rL245729 )
http://reviews.llvm.org/D10662 ( http://reviews.llvm.org/rL245075 )

This makes the attribute name match most of the existing lowering logic
and regression test expectations.

But the current use of this attribute is inconsistent; see the FIXME
comment for "allowsMisalignedMemoryAccesses()". That change will
result in functional changes and should be coming soon.

llvm-svn: 246585
diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index 7a37d4c..cf9d8a8 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -5511,7 +5511,7 @@
   // TODO: Check if 32-byte or greater accesses are slow too?
   if (!MI->hasOneMemOperand() &&
       RC == &X86::VR128RegClass &&
-      Subtarget.isUnalignedMemUnder32Slow())
+      Subtarget.isUnalignedMem16Slow())
     // Without memoperands, loadRegFromAddr and storeRegToStackSlot will
     // conservatively assume the address is unaligned. That's bad for
     // performance.
@@ -5659,7 +5659,7 @@
                             cast<MachineSDNode>(N)->memoperands_end());
     if (!(*MMOs.first) &&
         RC == &X86::VR128RegClass &&
-        Subtarget.isUnalignedMemUnder32Slow())
+        Subtarget.isUnalignedMem16Slow())
       // Do not introduce a slow unaligned load.
       return false;
     // FIXME: If a VR128 can have size 32, we should be checking if a 32-byte
@@ -5704,7 +5704,7 @@
                              cast<MachineSDNode>(N)->memoperands_end());
     if (!(*MMOs.first) &&
         RC == &X86::VR128RegClass &&
-        Subtarget.isUnalignedMemUnder32Slow())
+        Subtarget.isUnalignedMem16Slow())
       // Do not introduce a slow unaligned store.
       return false;
     // FIXME: If a VR128 can have size 32, we should be checking if a 32-byte