[i386] New lea patterns for PR71321

[Bernd Schmidt]

The problem here is that we don't have complete coverage of lea patterns 
for HImode/QImode: the combiner can't recognize a (plus (ashift reg 2) 
reg) pattern it builds.

My first idea was to canonicalize ASHIFT by constant inside PLUS to 
MULT. The docs say that this is done only inside a MEM context, but that 
seems misguided considering the existence of lea patterns. Maybe that's 
something to revisit for gcc-8. In the meantime, the patch below is more 
like a minimal fix, and it seems to produce better results at the moment 
anyway.

Bootstrapped and tested on x86_64-linux. Ok?


Bernd
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