Provide extra checking for phi argument access from edge
The following adds checking that the edge we query an associated
PHI arg for is related to the PHI node. Triggered by questionable
code in one of my reviews.
* gimple.h (gimple_phi_arg): New const overload.
(gimple_phi_arg_def): Make gimple arg const.
(gimple_phi_arg_def_from_edge): New inline function.
* tree-phinodes.h (gimple_phi_arg_imm_use_ptr_from_edge):
Likewise.
* tree-ssa-operands.h (PHI_ARG_DEF_FROM_EDGE): Direct to
new inline function.
(PHI_ARG_DEF_PTR_FROM_EDGE): Likewise.