[llvm] [LoongArch] Enable LoopTermFold Pass (PR #130737)

[via llvm-commits]

tangaac wrote:

> What's the effect after enabling this pass?

This will reduce one `addi` instruction in  loop.body but with extra setup cost.
For most cases, this keep the same behavior with gcc.

clang temp.c -S -O1
``` c
void foo(int *__restrict a, short int * restrict b, int n) {
	for(int i = 0 ; i < n; i++ )
		a[i] = b[i];
}
```
before 

```asm
# %bb.0:
	ori	$a3, $zero, 1
	blt	$a2, $a3, .LBB0_2
	.p2align	4, , 16
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
	ld.h	$a3, $a1, 0
	st.w	$a3, $a0, 0
	addi.d	$a0, $a0, 4
	addi.d	$a2, $a2, -1
	addi.d	$a1, $a1, 2
	bnez	$a2, .LBB0_1
.LBB0_2:
	ret
```

after
```asm
# %bb.0:                                # %entry
	ori	$a3, $zero, 1
	blt	$a2, $a3, .LBB0_3
# %bb.1:                                # %for.body.preheader
	alsl.d	$a2, $a2, $a0, 2
	.p2align	4, , 16
.LBB0_2:                                # %for.body
                                        # =>This Inner Loop Header: Depth=1
	ld.h	$a3, $a1, 0
	st.w	$a3, $a0, 0
	addi.d	$a0, $a0, 4
	addi.d	$a1, $a1, 2
	bne	$a0, $a2, .LBB0_2
.LBB0_3:                                # %for.cond.cleanup
	ret

```

https://github.com/llvm/llvm-project/pull/130737