Change SelectionDAG type legalization to allow BUILD_VECTOR operands to be

promoted to legal types without changing the type of the vector.  This is
following a suggestion from Duncan
(http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-February/019923.html).
The transformation that used to be done during type legalization is now
postponed to DAG legalization.  This allows the BUILD_VECTORs to be optimized
and potentially handled specially by target-specific code.

It turns out that this is also consistent with an optimization done by the
DAG combiner: a BUILD_VECTOR and INSERT_VECTOR_ELT may be combined by
replacing one of the BUILD_VECTOR operands with the newly inserted element;
but INSERT_VECTOR_ELT allows its scalar operand to be larger than the
element type, with any extra high bits being implicitly truncated.  The
result is a BUILD_VECTOR where one of the operands has a type larger the
the vector element type.

Any code that operates on BUILD_VECTORs may now need to be aware of the
potential type discrepancy between the vector element type and the
BUILD_VECTOR operands.  This patch updates all of the places that I could
find to handle that case.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68996 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -290,7 +290,11 @@ namespace ISD {
 
     /// BUILD_VECTOR(ELT0, ELT1, ELT2, ELT3,...) - Return a vector
     /// with the specified, possibly variable, elements.  The number of elements
-    /// is required to be a power of two.
+    /// is required to be a power of two.  The types of the operands must
+    /// all be the same.  They must match the vector element type, except if an 
+    /// integer element type is not legal for the target, the operands may 
+    /// be promoted to a legal type, in which case the operands are implicitly
+    /// truncated to the vector element types.
     BUILD_VECTOR,
 
     /// INSERT_VECTOR_ELT(VECTOR, VAL, IDX) - Returns VECTOR with the element