[RISCV] Set CostPerUse for registers

Summary:
 Set CostPerUse higher for registers that are not used in the compressed
 instruction set. This will influence the greedy register allocator to reduce
 the use of registers that can't be encoded in 16 bit instructions. This
 affects register allocation even when compressed instruction isn't targeted,
 we see no major negative codegen impact.

Reviewers: asb

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, jordy.potman.lists, apazos, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang

Differential Revision: https://reviews.llvm.org/D47039

llvm-svn: 333132