cmd/compile: allow ir.OSLICE2ARRPTR in mayCall

CL 301650 adds conversion from slice to array ptr. The conversion
expression may appear as argument to a function call, so it will be
tested by mayCall. But ir.OSLICE2ARRPTR  op is not handled by mayCall,
causes the compiler crashes.

Updates #395
Fixes #46720

Change-Id: I39e1b3e38e224a31f3dec46dbbdc855ff3b2c6a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/327649


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