i386.md bug + fix

[Jeffrey A Law]

  In message < 199805062200.XAA00275@wildcat.demon.co.uk >you write:
  > The strlensi pattern in i386.md is still well and truly
  > screwed due to a missing USE on an implicitly used register
  > within the strlensi define_expand and its associated unspec.
I was looking at this last night.  While I agree that you've
found a bug, I do not thing adding the USE is the right solution
at this time.

Thanks for providing a testcase this time, it will make analyzing
this problem much easier.

jeff