spirv-opt: clarify declaration of FindNumberOfIterations (KhronosGroup#6727)

This function takes the branch instruction, not the condition
instruction as returned by GetConditionInst(). Rename the parameter in
the header (the implementation already uses the right name) and update
the comment to save the next person discovering this a bit of time.

---------

Signed-off-by: Kevin Petit <kevin.petit@arm.com>
Co-authored-by: alan-baker <alanbaker@google.com>

Author: kpet

source/opt/loop_descriptor.h

@@ -242,12 +242,12 @@ class Loop {
   Instruction* FindConditionVariable(const BasicBlock* condition) const;
 
   // Returns the number of iterations within a loop when given the |induction|
-  // variable and the loop |condition| check. It stores the found number of
+  // variable and the loop |branch| instruction. It stores the found number of
   // iterations in the output parameter |iterations| and optionally, the step
-  // value in |step_value| and the initial value of the induction variable in
+  // value in |step_amount| and the initial value of the induction variable in
   // |init_value|.
   bool FindNumberOfIterations(const Instruction* induction,
-                              const Instruction* condition, size_t* iterations,
+                              const Instruction* branch, size_t* iterations,
                               int64_t* step_amount = nullptr,
                               int64_t* init_value = nullptr) const;