[Hexagon] Fix range checks for immediate operands

The output assembly (textual) contains the instruction
  r29 = add(r29,#4294967136)
The value 4294967136 is -160 when interpreted as a signed 32-bit
integer, so it fits in the range of the immediate operand without
a constant extender. The range check in HexagonInstrInfo was putting
the operand value into an int variable, reporting no need for an
extender. This resulted in a packet with 4 instructions, including
the "add". The corresponding check in HexagonMCInstrInfo was using
an int64_t variable, causing the range check to fail, and an extender
to be emitted when lowering to MCInst, resulting in a packet with
too many instructions.