rtl-ssa: Extend m_num_defs to a full unsigned int [PR108086]

rtl-ssa: Extend m_num_defs to a full unsigned int [PR108086]

insn_info tried to save space by storing the number of
definitions in a 16-bit bitfield.  The justification was:

  // ...  FIRST_PSEUDO_REGISTER + 1
  // is the maximum number of accesses to hard registers and memory, and
  // MAX_RECOG_OPERANDS is the maximum number of pseudos that can be
  // defined by an instruction, so the number of definitions should fit
  // easily in 16 bits.

But while that reasoning holds (I think) for real instructions,
it doesn't hold for artificial instructions.  I don't think there's
any sensible higher limit we can use, so this patch goes for a full
unsigned int.

gcc/
	PR rtl-optimization/108086
	* rtl-ssa/insns.h (insn_info): Make m_num_defs a full unsigned int.
	Adjust size-related commentary accordingly.

(cherry picked from commit cd41085a37b8288dbdfe0f81027ce04b978578f1)