LoopVectorize/test: fix opt invocations with -march (NFC) (llvm#74462)

opt accepts the -march command-line argument, but this argument only
makes sense in conjunction with -mtriple. Fix a couple of tests under
LoopVectorize that invoke opt with -march but without -mtriple, to avoid
confusing users.

Author: artagnon

llvm/test/Transforms/LoopVectorize/Hexagon/maximum-vf-crash.ll

@@ -1,12 +1,11 @@
-; RUN: opt -march=hexagon -hexagon-autohvx -passes=loop-vectorize -S < %s 2>&1 | FileCheck %s
+; RUN: opt -mtriple=hexagon -hexagon-autohvx -passes=loop-vectorize -S < %s 2>&1 | FileCheck %s
 
 ; Check that we don't crash.
 
 ; CHECK-LABEL: @f
 ; CHECK: vector.body
 
 target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
-target triple = "hexagon"
 
 ; Function Attrs: optsize
 define i32 @f() #0 {

llvm/test/Transforms/LoopVectorize/Hexagon/minimum-vf.ll

@@ -1,12 +1,11 @@
-; RUN: opt -march=hexagon -passes=loop-vectorize -hexagon-autohvx -debug-only=loop-vectorize -disable-output < %s 2>&1 | FileCheck %s
+; RUN: opt -mtriple=hexagon -passes=loop-vectorize -hexagon-autohvx -debug-only=loop-vectorize -disable-output < %s 2>&1 | FileCheck %s
 ; REQUIRES: asserts
 
 ; Check that TTI::getMinimumVF works. The calculated MaxVF was based on the
 ; register pressure and was less than 64.
 ; CHECK: LV: Overriding calculated MaxVF({{[0-9]+}}) with target's minimum: 64
 
 target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
-target triple = "hexagon"
 
 %s.0 = type { ptr, i32, i32, i32, i32 }