[ConstraintSystem] Stop type checking TapExpr elements in shrink()

[rintaro]

TapExpr has a VarDecl the type of which is determined by type checking the parent interpolated string literal expression. Type checking TapExpr elements before that always fails, thus a waste of the computing time.

[rintaro]

@swift-ci Please smoke test

[rintaro]

https://ci.swift.org/job/swift-PR-osx-smoke-test/23199/console

/Users/buildnode/jenkins/workspace/swift-PR-osx-smoke-test/branch-master/swift/test/Parse/pointer_conversion.swift.gyb:310:43: error: incorrect message found
  let a: Bool = x == y // expected-error{{referencing operator function '==' on 'Array' requires that 'NotEquatable' conform to 'Equatable'}}
                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                          operator function '==' requires that 'NotEquatable' conform to 'Equatable'

Not sure this is caused by the patch.

[xedin]

@rintaro I don't think it is, I'm still trying to track down why this happens...

[xedin]

@swift-ci please smoke test macOS platform