compile.c: support branch coverage for `a&.foo = 1`

	[Bug #15476]

	compile.c (iseq_set_sequence): fix branch coverage table

	Not only TRACE_ELEMENT but also INSN_ELEMENT may have events.
	The old pc2branchindex was created using only events of TRACE_ELEMENTs.
	This change uses events of INSN_ELEMENTs too for pc2branchindex table.

	[Bug #15476]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66813 b2dd03c8-39d4-4d8f-98ff-823fe69b080e