[PSI2IR] Propagate smart cast information for variable loads.

This gives us more precise type information and can enable backend
optimizations. This was motivated by when expressions not compiled
to table switches in the JVM_IR backend.

Fixed KT-36845.

compiler/testData/ir/irText/expressions/implicitCastToNonNull.txt

@@ -41,7 +41,7 @@ FILE fqName:<root> fileName:/implicitCastToNonNull.kt
           BRANCH
             if: CONST Boolean type=kotlin.Boolean value=true
             then: CALL 'public abstract fun <get-length> (): kotlin.Int declared in kotlin.CharSequence' type=kotlin.Int origin=GET_PROPERTY
-              $this: TYPE_OP type=kotlin.CharSequence origin=IMPLICIT_CAST typeOperand=kotlin.CharSequence
+              $this: TYPE_OP type=T of <root>.test3 origin=IMPLICIT_CAST typeOperand=T of <root>.test3
                 GET_VAR 'x: kotlin.Any declared in <root>.test3' type=kotlin.Any origin=null
   FUN name:test4 visibility:public modality:FINAL <T> (x:kotlin.Any?) returnType:kotlin.Int [inline]
     TYPE_PARAMETER name:T index:0 variance: superTypes:[kotlin.CharSequence]
@@ -56,7 +56,7 @@ FILE fqName:<root> fileName:/implicitCastToNonNull.kt
           BRANCH
             if: CONST Boolean type=kotlin.Boolean value=true
             then: CALL 'public abstract fun <get-length> (): kotlin.Int declared in kotlin.CharSequence' type=kotlin.Int origin=GET_PROPERTY
-              $this: TYPE_OP type=kotlin.CharSequence origin=IMPLICIT_CAST typeOperand=kotlin.CharSequence
+              $this: TYPE_OP type=T of <root>.test4 origin=IMPLICIT_CAST typeOperand=T of <root>.test4
                 GET_VAR 'x: kotlin.Any? declared in <root>.test4' type=kotlin.Any? origin=null
   FUN name:test5 visibility:public modality:FINAL <T, S> (x:T of <root>.test5, fn:kotlin.Function1<S of <root>.test5, kotlin.Unit>) returnType:kotlin.Unit
     TYPE_PARAMETER name:T index:0 variance: superTypes:[S of <root>.test5?]