cmd/compile/internal/inline/inlheur: delete ConstExpr case
ir.ConstExpr was deleted in CL 526395, so no need to check for it.
Fixes the build error.
Change-Id: I642dab70d17369bc7c9ae880666ef42ff099b770
Reviewed-on: https://go-review.googlesource.com/c/go/+/526196
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