rtlanal: Don't assume that calls write to a global SP [PR99596]

rtlanal: Don't assume that calls write to a global SP [PR99596]

This patch is a GCC 11 regression caused by the rtl-ssa code.
Normally we treat calls as containing a potential set of a global
register, but DF makes a sensible exception for the stack pointer:

      if (i == STACK_POINTER_REGNUM)
	/* The stack ptr is used (honorarily) by a CALL insn.  */
	df_ref_record (DF_REF_BASE, collection_rec, regno_reg_rtx[i],
		       NULL, bb, insn_info, DF_REF_REG_USE,
		       DF_REF_CALL_STACK_USAGE | flags);
      else if (global_regs[i])
	{
	  /* Calls to const functions cannot access any global registers and
	     calls to pure functions cannot set them.  All other calls may
	     reference any of the global registers, so they are recorded as
	     used. */

The only DF definition of SP was therefore the one in the entry block.
However, the rtlanal.c rtx_properties code (wrongly) assumed that calls
also clobbered the global SP.  This led to multiple definitions of SP
when we only expected one.

This patch tightens the rtlanal.c handling of global registers
to match the DF approach.

gcc/
	PR rtl-optimization/99596
	* rtlanal.c (rtx_properties::try_to_add_insn): Don't add global
	register accesses for const calls.  Assume that pure functions
	can only read from global registers.  Ignore cases in which
	the stack pointer has been marked global.

gcc/testsuite/
	PR rtl-optimization/99596
	* gcc.target/arm/pr99596.c: New test.