X86: Resolve a long standing FIXME and properly isel pextr[bw].

Generalize the AArch64 .td nodes for AssertZext and AssertSext. Use
them to match the relevant pextr store instructions.

The test widen_load-2.ll requires a slight change because with the
stores gone, the remaining instructions are scheduled in a different
order.

Add test cases for SSE4 and AVX variants.

Resolves rdar://13414672.

Patch by Adam Nemet <anemet@apple.com>.