[CostModel][X86][AArch64] Adjust cost of the scalarization part of min/max reduction.

Summary: The comment says we need 3 extracts and a select at the end. But didn't we just account for the select in the vector cost above. Aren't we just extracting the single element after taking the min/max in the vector register?

Reviewers: RKSimon, spatel, ABataev

Reviewed By: RKSimon

Subscribers: javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D55480

llvm-svn: 348739