xtensa: Fix remaining inaccuracies in xtensa_is_insn_L32R_p()

xtensa: Fix remaining inaccuracies in xtensa_is_insn_L32R_p()

The previous fix also had some flaws:

- The TARGET_CONST16 check was a bit premature
- It didn't take into account the possibility of the RTL expression
   "(set (reg:SF gpr) (const_int))", especially when TARGET_AUTOLITPOOLS is
   configured

This patch fixes the above.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_is_insn_L32R_p):
	Re-rewrite to more accurately capture insns that could be L32R machine
	instructions wherever possible, and add comments that help understand
	the intent of the process.