RFR: 8264104: Eliminate unnecessary vector mask conversion during VectorUnbox for floating point VectorMask [v2]

[Vladimir Ivanov]

On Thu, 8 Apr 2021 03:44:41 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>  I notice that VectorNode and its subclasses do not check for TOP inputs. 
> Since Vector API introduce vectors in graph before SuperWord transformation their input could become dead. 
> How such cases handled? And why we did not hit them yet? is_vect() should hit assert.

`VectorLoadMaskNode::Identity()` can't observe TOP types because it uses the type cached at construction (`type()` and `VectorNode` extends `TypeNode`). Still, a TOP input is possible and should be filtered out by opcode check (`in(1)->Opcode() == Op_VectorStoreMask`).

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PR: https://git.openjdk.java.net/jdk/pull/3238