Allow misaligned stores in x86 fast-isel.

In X86FastISel::X86SelectStore(), improperly aligned stores are rejected and
handled by the DAG-based ISel.  However, X86FastISel::X86SelectLoad() makes
no such requirement.  There doesn't appear to be an x86 architectural
correctness issue with allowing potentially unaligned store instructions.
This patch removes this restriction.

Patch by Jim Stichnot.