[llvm] [InstCombine] Fix unsafe PHINode cast and simplify logic in PointerReplacer (PR #172332)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-dev-x86-64` running on `ml-opt-dev-x86-64-b2` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/137/builds/30766

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/MLRegAlloc/dev-mode-log-2-fcts.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 6
/b/ml-opt-dev-x86-64-b1/build/bin/llc -mtriple=x86_64-linux-unknown -regalloc=greedy -regalloc-enable-advisor=development    -regalloc-training-log=/b/ml-opt-dev-x86-64-b1/build/test/CodeGen/MLRegAlloc/Output/dev-mode-log-2-fcts.ll.tmp1 < /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/test/CodeGen/MLRegAlloc/dev-mode-log-2-fcts.ll
# executed command: /b/ml-opt-dev-x86-64-b1/build/bin/llc -mtriple=x86_64-linux-unknown -regalloc=greedy -regalloc-enable-advisor=development -regalloc-training-log=/b/ml-opt-dev-x86-64-b1/build/test/CodeGen/MLRegAlloc/Output/dev-mode-log-2-fcts.ll.tmp1
# .---command stdout------------
# | 	.file	"<stdin>"
# | 	.section	.text.unlikely.,"ax", at progbits
# | 	.globl	f1                              # -- Begin function f1
# | 	.p2align	4
# | 	.type	f1, at function
# | f1:                                     # @f1
# | 	.cfi_startproc
# | # %bb.0:
# | 	pushq	%r14
# | 	.cfi_def_cfa_offset 16
# | 	pushq	%rbx
# | 	.cfi_def_cfa_offset 24
# | 	pushq	%rax
# | 	.cfi_def_cfa_offset 32
# | 	.cfi_offset %rbx, -24
# | 	.cfi_offset %r14, -16
# | 	movq	%rdx, %rbx
# | 	movq	%rdi, %r14
# | 	addq	%rsi, %r14
# | 	callq	f at PLT
# | 	movq	%r14, (%rbx)
# | 	addq	$8, %rsp
# | 	.cfi_def_cfa_offset 24
# | 	popq	%rbx
# | 	.cfi_def_cfa_offset 16
# | 	popq	%r14
# | 	.cfi_def_cfa_offset 8
# | 	retq
# | .Lfunc_end0:
# | 	.size	f1, .Lfunc_end0-f1
# | 	.cfi_endproc
# |                                         # -- End function
# | 	.section	.text.hot.,"ax", at progbits
# | 	.globl	f2                              # -- Begin function f2
# | 	.p2align	4
# | 	.type	f2, at function
# | f2:                                     # @f2
# | 	.cfi_startproc
# | # %bb.0:
# | 	addq	%rsi, %rdi
# | 	movq	%rdi, (%rdx)
...

```

</details>

https://github.com/llvm/llvm-project/pull/172332