[COMMITTED] tree-optimization/102738 - Simplification for right shift.

[Andrew MacLeod]

As the PR observes, if the first operand of a right shift is 0 or -1, 
operand 2 doesn't matter and the result will be the same as op1, so it 
can be turned into a copy.

This patch checks for that condition and performs the operation in EVRP.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew

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