KT-60954 [Analysis API] Unwrap variable assignments in KtFirReferenceShortener

To get to the proper qualified expression, we need to unwrap the outer
`FirVariableAssignment` if we deal with the property access inside of
assignment expressions

^KT-60954 Fixed

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment.kt

@@ -0,0 +1,11 @@
+// FILE: main.kt
+package test
+
+fun usage() {
+    <expr>dependency.foo = 20</expr>
+}
+
+// FILE: dependency.kt
+package dependency
+
+var foo: Int = 10

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment.txt

@@ -0,0 +1,7 @@
+Before shortening: dependency.foo = 20
+with DO_NOT_SHORTEN:
+with SHORTEN_IF_ALREADY_IMPORTED:
+with SHORTEN_AND_IMPORT:
+[qualifier] dependency.foo
+with SHORTEN_AND_STAR_IMPORT:
+[qualifier] dependency.foo

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment_plusAssignOperator.kt

@@ -0,0 +1,15 @@
+// FILE: main.kt
+package test
+
+fun usage() {
+    <expr>dependency.foo += 20</expr>
+}
+
+// FILE: dependency.kt
+package dependency
+
+val foo: Foo = Foo()
+
+class Foo {
+    operator fun plusAssign(i: Int) {}
+}

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment_plusAssignOperator.txt

@@ -0,0 +1,7 @@
+Before shortening: dependency.foo += 20
+with DO_NOT_SHORTEN:
+with SHORTEN_IF_ALREADY_IMPORTED:
+with SHORTEN_AND_IMPORT:
+[qualifier] dependency.foo
+with SHORTEN_AND_STAR_IMPORT:
+[qualifier] dependency.foo

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment_plusOperator.kt

@@ -0,0 +1,15 @@
+// FILE: main.kt
+package test
+
+fun usage() {
+    <expr>dependency.foo += 20</expr>
+}
+
+// FILE: dependency.kt
+package dependency
+
+var foo: Foo = Foo()
+
+class Foo {
+    operator fun plus(i: Int): Foo = this
+}

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/variableAssignment_plusOperator.txt

@@ -0,0 +1,7 @@
+Before shortening: dependency.foo += 20
+with DO_NOT_SHORTEN:
+with SHORTEN_IF_ALREADY_IMPORTED:
+with SHORTEN_AND_IMPORT:
+[qualifier] dependency.foo
+with SHORTEN_AND_STAR_IMPORT:
+[qualifier] dependency.foo