[AArch64][SVE] Allow to lower WHILEop with constant operands to PTRUE

This allows it to fold WHILEop with constant operand to PTRUE instruction in
the case given range is fitted to predicate format. Also, this change
fixes the unsigned overflow error introduced in D137547 for WHILELO lowering.

Differential Revision: https://reviews.llvm.org/D139068