gimple ssa: switchconv: Use __builtin_popcount and support more types in exp transform [PR116355]

The gen_pow2p function generates (a & -a) == a as a fallback for
POPCOUNT (a) == 1.  Not only is the bitmagic not equivalent to
POPCOUNT (a) == 1 but it also introduces UB (consider signed
a = INT_MIN).

This patch rewrites gen_pow2p to always use __builtin_popcount instead.
This means that what the end result GIMPLE code is gets decided by an
already existing machinery in a later pass.  That is a cleaner solution
I think.  This existing machinery also uses a ^ (a - 1) > a - 1 which is
the correct bitmagic.

While rewriting gen_pow2p I had to add logic for converting the
operand's type to a type that __builtin_popcount accepts.  I naturally
also added this logic to gen_log2.  Thanks to this, exponential index
transform gains the capability to handle all operand types with
precision at most that of long long int.

gcc/ChangeLog:

	PR tree-optimization/116355
	* tree-switch-conversion.cc (can_log2): Add capability to
	suggest converting the operand to a different type.
	(gen_log2): Add capability to generate a conversion in case the
	operand is of a type incompatible with the logarithm operation.
	(can_pow2p): New function.
	(gen_pow2p): Rewrite to use __builtin_popcount instead of
	manually inserting an internal fn call or bitmagic.  Also add
	capability to generate a conversion.
	(switch_conversion::is_exp_index_transform_viable): Call
	can_pow2p.  Store types suggested by can_log2 and gen_log2.
	(switch_conversion::exp_index_transform): Params of gen_pow2p
	and gen_log2 changed so update their calls.
	* tree-switch-conversion.h: Add m_exp_index_transform_log2_type
	and m_exp_index_transform_pow2p_type to switch_conversion class
	to track type conversions needed to generate the "is power of 2"
	and logarithm operations.

gcc/testsuite/ChangeLog:

	PR tree-optimization/116355
	* gcc.target/i386/switch-exp-transform-1.c: Don't test for
	presence of POPCOUNT internal fn after switch conversion.  Test
	for it after __builtin_popcount has had a chance to get
	expanded.
	* gcc.target/i386/switch-exp-transform-3.c: Also test char and
	short.

Signed-off-by: Filip Kastl <fkastl@suse.cz>