[llvm] [LLVM][IR] Add textual shorthand for specifying constant vector splats. (PR #74620)

[Paul Walker via llvm-commits]

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@@ -3981,6 +3981,21 @@ bool LLParser::parseValID(ValID &ID, PerFunctionState *PFS, Type *ExpectedTy) {
     return false;
   }
 
+  case lltok::kw_splat: {
+    Lex.Lex();
+    if (parseToken(lltok::lparen, "expected '(' after vector splat"))
+      return true;
+    Constant *C;
+    if (parseGlobalTypeAndValue(C))
+      return true;
+    if (parseToken(lltok::rparen, "expected ')' at end of vector splat"))
+      return true;
+
+    ID.ConstantVal = C;
+    ID.Kind = ValID::t_ConstantSplat;
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paulwalker-arm wrote:

We don’t have the target type at this point in the parsing (NOTE `ExpectedTy` defaults to `nullptr` and is only used in one part of `parseValID`).  ConstantExprs can be constructed based on their input operands, with only their final result type needing to be validated within `convertValIDToValue`. 

The reason for not reusing `t_Constant` is to maintain the accuracy of the diagnostics.

https://github.com/llvm/llvm-project/pull/74620