[PR rtl-optimization/20532] plus(ashift,ashift) -> mult may overflow

[Roger Sayle]

On 22 Mar 2005, Alexandre Oliva wrote:
>	PR rtl-optimization/20532
>	* simplify-rtx.c (simplify_binary_operation_1): Protect from
>	overflow when adding coefficients for PLUS or MINUS.
>	(simplify_binary_operation_1): Handle CONST_DOUBLE exact power of
>	two as multiplier.

This is OK for mainline once regression testing on amd64-linux-gnu
completes without problems.  After two or three days on mainline without
any reported problems, this patch is also OK for the 4.0 branch (after
bootstrapping and regression testing there).

Many thanks for fixing this.

Roger
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