[Test] Separate IndVars test into AArch64 and X86 parts

[Test] Separate IndVars test into AArch64 and X86 parts

The widen-loop-comp.ll in indvars has a target triple with
specified aarch64 architecture. This caused test failures with
db28934 "[IndVars] Pass TTI to replaceCongruentIVs" applied, because
with the patch indvars performed some target-specific
transforms, and for example if a build supported only X86,
then indvars would not have applied those transforms.
However, the checks in the test were generated as for aarch64.
Thus the test failures on such builds.

This patch separates widen-loop-comp.ll into two parts.
The first one is intended to be run only if a build supports aarch64.
This is now in AArch64 directory with a lit config.

The second one was added recently to show db28934 improvements.
This one is now in X86 directory.

This patch should resolve build issues caused by
5ec23863320ca12bfabb6dcff1d0425cb614b7a5.