[RISCV] Fix RISCVTTIImpl::getIntImmCost for immediates where getMinSignedBits() > 64

APInt::getSExtValue will assert if getMinSignedBits() > 64. This can happen,
for instance, if examining an i128. Avoid this assertion by checking
Imm.getMinSignedBits() <= 64 before doing
getTLI()->isLegalAddImmediate(Imm.getSExtValue()). We could directly check
getMinSignedBits() <= 12 but it seems better to reuse the isLegalAddImmediate
helper for this.

Differential Revision: https://reviews.llvm.org/D64390

llvm-svn: 365462