Set discriminators for call stmts on the same line within the same basic block.

Set discriminators for call stmts on the same line within the same basic block.

Call statements are possible split points of a basic block so they may end up
in different basic blocks by the time pass_ipa_auto_profile executes.

This change will also simplify call site lookups since now location with discriminator
will uniquely identify the call site (no callee function name is needed).

This change is based on commit 1e6c4a7a8fb8e20545bb9f9032d3854f3f794c18
by Dehao Chen in vendors/google/heads/gcc-4_8.

Tested on x86_64-pc-linux-gnu.

gcc/ChangeLog:
	* tree-cfg.cc (assign_discriminators): Set discriminators for call stmts
	on the same line within the same basic block.