[llvm] [FMF] Set all bits if needed when setting individual flags. (PR #131321)

[Matt Arsenault via llvm-commits]

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@@ -75,24 +77,31 @@ class FastMathFlags {
   /// Flag setters
   void setAllowReassoc(bool B = true) {
     Flags = (Flags & ~AllowReassoc) | B * AllowReassoc;
+    setAllBitsIfNeeded();
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arsenm wrote:

Wouldn't it be easier to fix all to check Flags & Fullmask == FullMask? 

https://github.com/llvm/llvm-project/pull/131321