A pattern to simplify WhereOp

[tungld]

This patch adds a canonicalization rule to WhereOp to simplify the following pattern (found in the model xlm-roberta-base-language-detection):

    %0 = onnx.Constant dense<-1> : tensor<2xi64>
    %1 = onnx.Constant dense<1> : tensor<2xi64>
    %2 = "onnx.Dim"(%arg0) {axis = 0 : si64} : (tensor<?x?xi64>) -> tensor<1xi64>
    %3 = "onnx.Dim"(%arg0) {axis = 1 : si64} : (tensor<?x?xi64>) -> tensor<1xi64>
    %4 = "onnx.Concat"(%2, %3) {axis = 0 : si64} : (tensor<1xi64>, tensor<1xi64>) -> tensor<2xi64>
    %5 = "onnx.Equal"(%4, %0) : (tensor<2xi64>, tensor<2xi64>) -> tensor<2xi1>
    %6 = "onnx.Where"(%5, %1, %4) : (tensor<2xi1>, tensor<2xi64>, tensor<2xi64>) -> tensor<2xi64>

Since the condition of where, i.e. %5, is always false (it compares dimension sizes with -1, and dimension sizes are always positive so the result is false), onnx.Where can be replaced by its "false" value that is %4.
into

    %0 = "onnx.Dim"(%arg0) {axis = 0 : si64} : (tensor<?x?xi64>) -> tensor<1xi64>
    %1 = "onnx.Dim"(%arg0) {axis = 1 : si64} : (tensor<?x?xi64>) -> tensor<1xi64>
    %2 = "onnx.Concat"(%0, %1) {axis = 0 : si64} : (tensor<1xi64>, tensor<1xi64>) -> tensor<2xi64>

This simplification of WhereOp helps DimAnalysis work well with the model xlm-roberta-base-language-detection so that all MatMuls could run on NNPA.

[AlexandreEichenberger]

LGTM, outstanding catch.

[tungld]

@jenkins-droid test this please

[jenkins-droid]

Jenkins Linux s390x Build #14786 [push] A pttern to simplify Whe... started at 02:38

[jenkins-droid]

Jenkins Linux ppc64le Build #13811 [push] A pttern to simplify Whe... started at 02:50

[jenkins-droid]

Jenkins Linux amd64 Build #14781 [push] A pttern to simplify Whe... started at 01:38

[jenkins-droid]

Jenkins Linux amd64 Build #14781 [push] A pttern to simplify Whe... passed after 1 hr 19 min

[jenkins-droid]

Jenkins Linux s390x Build #14786 [push] A pttern to simplify Whe... passed after 1 hr 54 min

[jenkins-droid]

Jenkins Linux ppc64le Build #13811 [push] A pttern to simplify Whe... passed after 2 hr 8 min