[NFC] Renamed /test/Analysis/CostModel/X86/splat-load.ll test and added more checks.

Renamed test/Analysis/CostModel/X86/splat-load.ll to shuffle-load.ll
to align it with AArch64's similar test.

Also added a complete list of checks for all vector combinations up to 512-bits.

Differential Revision: https://reviews.llvm.org/D124528

Author: vporpo