[PATCH] Teach Instcombine to use the nsw attribute for signed comparisons to 0 of (shl %v, Cst) or (mul %v, Cst)

[Arnaud de Grandmaison]

The attached patch teaches InstCombine/visitICmpInstWithInstAndIntCst to 
optimize 'icmp pred (mul %v, Cst), 0' when the mul has the nsw attribute set.
 The comparison can the be performed on the mul RHS. 

The patch also handles the case of the mul in strength reduced form : shl

Cheers
--
Arnaud de Grandmaison
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