This turned out to be useful for the SVE PCS support, and is a natural
tree-level analogue of insn_callee_abi.

2019-10-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* function-abi.h (expr_callee_abi): Declare.
	* function-abi.cc (expr_callee_abi): New function.

From-SVN: r276952