Add if-chain to switch conversion pass.

Add if-chain to switch conversion pass.

gcc/ChangeLog:

	PR tree-optimization/14799
	PR ipa/88702
	* Makefile.in: Add new gimple-if-to-switch.o.
	* common.opt: Add new option.
	* dbgcnt.def (DEBUG_COUNTER): Add new debug counter.
	* doc/invoke.texi: Document -fconvert-if-to-switch.
	* opts.c: Add -fconvert-if-to-switch at OPT_LEVELS_2_PLUS.
	* passes.def: Register new pass.
	* timevar.def (TV_TREE_IF_TO_SWITCH): Add new time variable.
	* tree-pass.h (make_pass_if_to_switch): New.
	* tree-switch-conversion.h: New file.
	* gimple-if-to-switch.cc: New file.

gcc/testsuite/ChangeLog:

	PR tree-optimization/14799
	PR ipa/88702
	* gcc.dg/tree-ssa/reassoc-32.c: Add -fno-convert-if-to-switch.
	* gcc.dg/tree-ssa/if-to-switch-1.c: New test.
	* gcc.dg/tree-ssa/if-to-switch-2.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-3.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-4.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-5.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-6.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-7.c: Likewise.
	* gcc.dg/tree-ssa/if-to-switch-8.c: Likewise.