Check if operands are immediates before trying shortening.

Summary:
Operands in the initial instruction stream should all have immediate operands
for instructions that can be shortened.  But if a BOLT optimization pass adds
one of these instructions with a symbolic operand, the shortening operation
will assert.  This diff adds checks to make sure that the operands are
immediate.

I've also disabled shortening pass by default since it won't really be needed
until ICP is submitted.  It will still run at CFG creation time.

(cherry picked from FBD3610646)