[InstCombine] Precommit test exhibiting miscompile

InstCombine is determining incorrect byte offsets for GEPs
into vectors of overaligned elements.
Add a new testcase showing this behavior, serving as precommit
for a fix.

Author: jasilvanus

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -1,7 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
-target datalayout = "e-p:64:64-p1:16:16-p2:32:32:32-p3:64:64:64"
+target datalayout = "e-p:64:64-p1:16:16-p2:32:32:32-p3:64:64:64-f16:32"
 
 %intstruct = type { i32 }
 %pair = type { i32, i32 }
@@ -111,6 +111,20 @@ define void @test_evaluate_gep_as_ptrs_array(ptr addrspace(2) %B) {
   ret void
 }
 
+define void @test_overaligned_vec(i8 %B) {
+        ; This should be turned into a constexpr instead of being an instruction
+; CHECK-LABEL: @test_overaligned_vec(
+; TODO: In this test case, half is overaligned to 32 bits.
+;       Vectors are bit-packed and don't respect alignment.
+;       Thus, the byte offset of the second half in <2 x half> is 2 bytes, not 4 bytes:
+; CHECK-NEXT:    store i8 [[B:%.*]], ptr getelementptr inbounds ([10 x i8], ptr @Global, i64 0, i64 4), align 1
+; CHECK-NEXT:    ret void
+;
+  %A = getelementptr <2 x half>, ptr @Global, i64 0, i64 1
+  store i8 %B, ptr %A
+  ret void
+}
+
 define ptr @test7(ptr %I, i64 %C, i64 %D) {
 ; CHECK-LABEL: @test7(
 ; CHECK-NEXT:    [[A:%.*]] = getelementptr i32, ptr [[I:%.*]], i64 [[C:%.*]]