[x86] Fix maskload/store intrinsic definitions in avxintrin.h

According to the Intel documentation, the mask operand of a maskload and
maskstore intrinsics is always a vector of packed integer/long integer values.
This patch introduces the following two changes:
 1. It fixes the avx maskload/store intrinsic definitions in avxintrin.h.
 2. It changes BuiltinsX86.def to match the correct gcc definitions for avx
    maskload/store (see D13861 for more details).

Differential Revision: http://reviews.llvm.org/D13861

llvm-svn: 250816