[InstCombine] Add test for -expensive-combines option; NFC

This shows that -expensive-combines=0 is ignored.

(cherry picked from commit 2d0d423)

Author: nikic

llvm/test/Transforms/InstCombine/expensive-combines.ll

@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -instcombine < %s | FileCheck %s --check-prefix=DEFAULT
+; RUN: opt -S -instcombine -expensive-combines=1 < %s | FileCheck %s --check-prefix=EXPENSIVE-ON
+; RUN: opt -S -instcombine -expensive-combines=0 < %s | FileCheck %s --check-prefix=EXPENSIVE-OFF
+
+define void @test() {
+; DEFAULT-LABEL: @test(
+; DEFAULT-NEXT:    [[CALL:%.*]] = call i32 @passthru(i32 0)
+; DEFAULT-NEXT:    call void @sink(i32 0)
+; DEFAULT-NEXT:    ret void
+;
+; EXPENSIVE-ON-LABEL: @test(
+; EXPENSIVE-ON-NEXT:    [[CALL:%.*]] = call i32 @passthru(i32 0)
+; EXPENSIVE-ON-NEXT:    call void @sink(i32 0)
+; EXPENSIVE-ON-NEXT:    ret void
+;
+; EXPENSIVE-OFF-LABEL: @test(
+; EXPENSIVE-OFF-NEXT:    [[CALL:%.*]] = call i32 @passthru(i32 0)
+; EXPENSIVE-OFF-NEXT:    call void @sink(i32 0)
+; EXPENSIVE-OFF-NEXT:    ret void
+;
+  %call = call i32 @passthru(i32 0)
+  call void @sink(i32 %call)
+  ret void
+}
+
+declare i32 @passthru(i32 returned)
+declare void @sink(i32)