[WinEH] Add an EH registration and state insertion pass for 32-bit x86

This pass is responsible for constructing the EH registration object
that gets linked into fs:00, which is all it does in this change. In the
future, it will also insert stores to update the EH state number.

I considered keeping this functionality in WinEHPrepare, but it's pretty
separable and X86 specific. It has conceptually very little to do with
the task of WinEHPrepare, which is currently outlining.  WinEHPrepare is
also in theory useful on ARM, but this logic is pretty x86 specific.

Reviewers: andrew.w.kaylor, majnemer

Differential Revision: http://reviews.llvm.org/D9422

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236339 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCAsmInfo.h

@@ -37,6 +37,7 @@ enum class EncodingType {
   ARM,     /// Windows NT (Windows on ARM)
   CE,      /// Windows CE ARM, PowerPC, SH3, SH4
   Itanium, /// Windows x64, Windows Itanium (IA-64)
+  X86,     /// Windows x86, uses no CFI, just EH tables
   MIPS = Alpha,
 };
 }
@@ -506,12 +507,13 @@ public:
   /// frame information to unwind.
   bool usesCFIForEH() const {
     return (ExceptionsType == ExceptionHandling::DwarfCFI ||
-            ExceptionsType == ExceptionHandling::ARM ||
-            ExceptionsType == ExceptionHandling::WinEH);
+            ExceptionsType == ExceptionHandling::ARM || usesWindowsCFI());
   }
 
   bool usesWindowsCFI() const {
-    return ExceptionsType == ExceptionHandling::WinEH;
+    return ExceptionsType == ExceptionHandling::WinEH &&
+           (WinEHEncodingType != WinEH::EncodingType::Invalid &&
+            WinEHEncodingType != WinEH::EncodingType::X86);
   }
 
   bool doesDwarfUseRelocationsAcrossSections() const {