[drreg] Omit adjacent tool-val restore and spill

Today, drreg restores app value of a spilled reg around app instrs that read that reg. E.g. for instr1 and instr2 that both read reg1, drreg will produce a sequence like the following:

<tool val spill for reg1>
<app val restore for reg1>
<app instr 1>
<tool val restore for reg1> // can be omitted

<tool val spill for reg1> // can be omitted
<app val restore for reg1>
<app instr 2>
<tool val restore for reg1>

The commented tool val restore and tool val spill instrs can possibly be omitted. This can be done in instru2instru phase in drreg.