Go patch committed: Don't evaluate interface conversions multiple times

[Ian Lance Taylor <iant at golang dot org>]

There were a few cases where the Go compiler was evaluating value
multiple times when they were converted to or from interface types.
This happened when the conversion occurred in a composite literal, or
even, when implicitly converting from one interface type to another,
in a variable declaration.  For this patch I added an assertion for
the cases where a value might be evaluated multiple times, and then
patched the compiler until it stopped crashing on the testsuite.
Bootstrapped and ran Go testsuite on x86_64-unknown-linux-gnu.
Committed to mainline.  This bug does not occur on the 4.9 branch.
I've committed a test case to the master Go repository
(https://go-review.googlesource.com/#/c/1710/).

Ian

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