Issue 8543 - [CTFE] simd literals need better support

https://d.puremagic.com/issues/show_bug.cgi?id=8543

Vector expressions in D are represented as ```cast(T[N])exp``` - this change lets CTFE be able to run interpret on the expression to expand it to it's underlying array literal.

Will add a test to the testsuite once have confirmed that there is no regressions from the autotester.