Revert r328386 "[X86] Fix Windows `i1 zeroext` conventions to use i8 instead of i32"

Revert r328386 "[X86] Fix Windows `i1 zeroext` conventions to use i8 instead of i32"

This broke Chromium (see crbug.com/825748). It looks like mstorsjo's follow-up
patch at D44876 fixes this, but let's revert back to green for now until that's
ready to land.

(Also reverts r328443.)

> Both GCC and MSVC only look at the low byte of a boolean when it is
> passed.

llvm-svn: 328482