Adding simple cast cost to ARM

Changing ARMBaseTargetMachine to return ARMTargetLowering intead of
the generic one (similar to x86 code).

Tests showing which instructions were added to cast when necessary
or cost zero when not. Downcast to 16 bits are not lowered in NEON,
so costs are not there yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173849 91177308-0d34-0410-b5e6-96231b3b80d8