[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/cast/InstanceEquals.box.txt

@@ -5,8 +5,8 @@ assert(null is String)
 
 Assertion failed
 assert(!(" is " is String))
-       |   |
-       |   true
+       |        |
+       |        true
        false
 
 Assertion failed
@@ -14,22 +14,22 @@ assert(!(
        |
        false
     " is "
-      |
-      true
 
             is
+            |
+            true
 
             String
 ))
 
 Assertion failed
 assert(null/*is*/is/*is*/String)
-       |
-       false
+            |
+            false
 
 Assertion failed
 assert(!((null is String) is Boolean))
-       |       |
-       |       true
+       |       |          |
+       |       |          true
        |       false
        false

plugins/power-assert/testData/codegen/cast/InstanceNotEquals.box.txt

@@ -5,27 +5,27 @@ assert("Hello, world!" !is String)
 
 Assertion failed
 assert(" !is " !is String)
-         |
-         false
+               |
+               false
 
 Assertion failed
 assert(
     " !is "
-      |
-      false
 
             !is
+            |
+            false
 
             String
 )
 
 Assertion failed
 assert("Hello, world!"/*!is*/!is/*!is*/String)
-       |
-       false
+                       |
+                       false
 
 Assertion failed
 assert(("Hello, world!" !is String) !is Boolean)
-                        |
-                        false
+                        |           |
+                        |           false
                         false