[PowerAssert] Correctly align infix calls for built-in operators

Instead of searching for the operator in the string representation of
the whole expression, consider the operator's start to be the
first non-whitespace non-dot character _after_ the LHS of the infix
expression.

This fixes cases like this:
```
assert("Name in " in listOf("Hello", "World"))
             |       |
             |       [Hello, World]
             false
```

^KT-66208 Fixed

plugins/power-assert/testData/codegen/operator/ExcleqeqOperator.box.txt

@@ -2,36 +2,30 @@ Assertion failed
 assert(1 !== 1)
          |
          false
-         true
 
 Assertion failed
 assert(" !== " !== " !== ")
-         |
-         false
-         true
+               |
+               false
 
 Assertion failed
 assert(
     " !== "
-      |
-      false
-      true
 
             !==
+            |
+            false
 
             " !== "
 )
 
 Assertion failed
 assert(1/*!==*/!==/*!==*/1)
-          |
-          false
-          true
+         |
+         false
 
 Assertion failed
 assert((1 !== 1) !== false)
-          |
-          false
-          true
-          false
-          true
+          |      |
+          |      false
+          false