avx1-i32x4 target

The changes contain the following:
- avx1-i32x4 target, which is sse4-i32x4 target with avx target-feature attribute. This is sufficient, see comments in code for more details. This target is better for integer workloads on AV in some cases.
- Some syntactic massaging of target-[sse4|avx].ll and target-[sse4|avx]-common.ll. The changes are reordering stuff and changing some comment. The purpose is to make files easily diffable. The only real minor change is adding a few of "alwaysinline" attributes to SSE4 version.
- example Makefiles are changed to accept any target via ISPC_IA_TARGET, not only predefined set. Single target now works well also.
- perf.py/alloy.py changes make perf.py run requested target with --target switch.
- fix for the bug when --target=avx1.1-i32x8,avx2-i32x8 - the problem was in the "target" for auto-dipatch function. In this case if was avx11, which is not valid target anymore.