Find more shrink-wrapping opportunities

[Bernd Schmidt]

This adds a little mini-pass to shrink-wrapping, to eliminate a common
case that often makes shrink-wrapping unavailable. If a move insn copies
an argument registers to a call-saved register, the prologue must be
emitted before this insn. We should therefore try to delay such moves
for as long as possible.

A followup patch will add an extra cprop pass, which replaces uses of
the call-saved register with uses of the incoming argument register,
thereby allowing us to move the copies in even more cases.

Bootstrapped and tested on i686-linux. Ok?


Bernd
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