X86: Custom lower sext v16i8 to v16i16, and the corresponding truncate.

Also update the cost model.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193270 91177308-0d34-0410-b5e6-96231b3b80d8
Reduce casting and use a fully covered switch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193272 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed doxygen comment to match Module.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193273 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Calling _chkstk is required on ELF as well as COFF on Windows. Without _chkstk functions requiring large stack crash in initialization code. Previous code tested for COFF format but not Mach-O and this patch modifies the code to test for Windows."

This reverts commit r193263.

It is causing CodeGen/X86/mingw-alloca.ll to fail.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193275 91177308-0d34-0410-b5e6-96231b3b80d8