[ARM] Mark 255 and 65535 as cheap for Thumb1 "And"

This prevents Constant Hoisting from pulling the constant out of the block,
allowing us to still produce LDRH/UXTH nodes. LDRB/UXTB (255) is already cheap
by the default getIntImmCost, but I've added it for clarity.

Differential Revision: https://reviews.llvm.org/D57671

llvm-svn: 353040