[runtime] Optimizations fixes for OP_IL_SEQ_POINT compatibility.

JIT optimizations were broken by the introduction of OP_IL_SEQ_POINT in the middle of the IR code.

The issues were fixed by modifying some `MonoInst *ins` and  `MonoBasicBlock *bb ` field usages to ignore OP_IL_SEQ_POINT. This was done with new helper methods:
 - `mono_inst_next (ins, FILTER_IL_SEQ_POINT)` replaces `ins->next` 
 - `mono_inst_prev (ins, FILTER_IL_SEQ_POINT)` replaces `ins->prev`
 - `mono_bb_first_inst (bb, FILTER_IL_SEQ_POINT)` replaces `bb->code`
 - `mono_bb_last_inst (bb, FILTER_IL_SEQ_POINT)` replaces `bb->last_ins`

Added test_op_il_seq_point.sh, it checks that each method of tested assemblies is equal with and without sequence points enabled. 
For now the test just compare methods size, which can be problem when an optimization does not change the size of the method. 
test_op_il_seq_point.sh will updated to compare the generated native code in an upcoming commit.