[NFCI][SimpleLoopUnswitch] Adjust CostKind query

When getUserCost was transitioned to use an explicit CostKind,
TCK_CodeSize was used even though the original kind was implicitly
SizeAndLatency so restore this behaviour. We now only query for
CodeSize when optimising for minsize.

I expect this to not change anything as, I think all, targets will
currently return the same value for CodeSize and SizeLatency. Indeed
I see no changes in the test suite for Arm, AArch64 and X86.

Differential Revision: https://reviews.llvm.org/D85829