add test ensuring simd codegen checks don't run when a static assertion failed

tests/ui/simd/const-err-trumps-simd-err.rs

@@ -0,0 +1,24 @@
+//@build-fail
+//! Make sure that monomorphization-time const errors from `static_assert` take priority over the
+//! error from simd_extract. Basically this checks that if a const fails to evaluate in some
+//! function, we don't bother codegen'ing the function.
+#![feature(generic_arg_infer)]
+#![feature(core_intrinsics)]
+#![feature(repr_simd)]
+#![feature(inline_const)]
+use std::intrinsics::simd::*;
+
+#[repr(simd)]
+#[allow(non_camel_case_types)]
+struct int8x4_t(u8,u8,u8,u8);
+
+fn get_elem<const LANE: u32>(a: int8x4_t) -> u8 {
+    const { assert!(LANE < 4); } // the error should be here...
+    //~^ ERROR failed
+    //~| assertion failed
+    unsafe { simd_extract(a, LANE) } // ...not here
+}
+
+fn main() {
+    get_elem::<4>(int8x4_t(0,0,0,0));
+}

tests/ui/simd/const-err-trumps-simd-err.stderr

@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `get_elem::<4>::{constant#0}` failed
+  --> $DIR/const-err-trumps-simd-err.rs:16:13
+   |
+LL |     const { assert!(LANE < 4); } // the error should be here...
+   |             ^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: LANE < 4', $DIR/const-err-trumps-simd-err.rs:16:13
+   |
+   = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn get_elem::<4>`
+  --> $DIR/const-err-trumps-simd-err.rs:23:5
+   |
+LL |     get_elem::<4>(int8x4_t(0,0,0,0));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.