Add an assertion: the zeroed_hardregs set is a subset of all call used regs.

Add an assertion: the zeroed_hardregs set is a subset of all call used regs.

We should make sure that the hard register set that is actually cleared by
the target hook zero_call_used_regs should be a subset of all call used
registers.

At the same time, update documentation for the target hook
TARGET_ZERO_CALL_USED_REGS.

This new assertion identified a bug in the i386 implemenation, which
incorrectly set the zeroed_hardregs for stack registers. Fixed this bug
in i386 implementation.

gcc/ChangeLog:

2022-04-01  Qing Zhao  <qing.zhao@oracle.com>

	* config/i386/i386.cc (zero_all_st_registers): Return the value of
	num_of_st.
	(ix86_zero_call_used_regs): Update zeroed_hardregs set according to
	the return value of zero_all_st_registers.
	* doc/tm.texi: Update the documentation of TARGET_ZERO_CALL_USED_REGS.
	* function.cc (gen_call_used_regs_seq): Add an assertion.
	* target.def: Update the documentation of TARGET_ZERO_CALL_USED_REGS.