bitint: Avoid extending ABI-extended large/huge _BitInts on load

bitint: Avoid extending ABI-extended large/huge _BitInts on load

This patch also make casts of ABI-extended large/huge _BitInts
behave the same as the small/middle case, i.e. no-op for casts
to a higher precision _BitInt with the same number of limbs /
extension for casts that turns a full top limb into a partial limb.
This conveniently helps keep some code with implementation-specific
extension semantics (e.g. BEXTC from gcc.dg/bitintext.h) the same
for _BitInts of any precision.

	* gimple-lower-bitint.cc (bitint_large_huge::limb_access):
	Add a parameter abi_load_p.  If set, load a limb directly
	in its actual precision without casting from m_limb_type.
	(struct bitint_large_huge): Same.
	(bitint_large_huge::handle_load): Use.