[X86] Fix pattern match for 32-to-64-bit zext in the presence of AssertSext

This fixes an issue with matching trunc -> assertsext -> zext on x86-64, which would not zero the high 32-bits. See PR20494 for details.
Recommitting - This time, with a hopefully working test.

Differential Revision: http://reviews.llvm.org/D6128