FIR LL API: resolve PSI elements to more specific FIR element

Compound access and implicit delegated constructor are mapped to FIR
elements that are way too broad.

analysis/low-level-api-fir/testdata/getOrBuildFir/calls/compoundAssignOnVal_lhs.kt

@@ -0,0 +1,6 @@
+interface A {
+    operator fun plusAssign(i: Int)
+}
+fun test(l: A) {
+    <expr>l</expr> += 1
+}