In IR or C code, shift amount larger than value size is undefined
behavior. But in practice, backend lowering for shift_parts produces
add/sub of shift amounts, thus constant shift amounts might be
negative or larger than value size, which depends on ISA definition.
PowerPC ISA says, the lowest 7 bits (6 bits for 32-bit instruction)
will be taken, and if the highest among them is 1, result will be
zero, otherwise the low 6 bits (or 5 on 32-bit) are used as shift
amount.
This commit emulates the behavior and avoids array overflow in bit
permutation's value bits calculator.
