[llvm] [AMDGPU][GlobalISel] Add RegBankLegalize support for G_MUL (PR #177314)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-devrel-x86-64` running on `ml-opt-devrel-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/175/builds/32303

<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/AMDGPU/GlobalISel/mul.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 2
/b/ml-opt-devrel-x86-64-b1/build/bin/llc -global-isel -new-reg-bank-select -mtriple=amdgcn -mcpu=gfx700 < /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll | /b/ml-opt-devrel-x86-64-b1/build/bin/FileCheck -check-prefixes=GCN,GFX7 /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll
# executed command: /b/ml-opt-devrel-x86-64-b1/build/bin/llc -global-isel -new-reg-bank-select -mtriple=amdgcn -mcpu=gfx700
# executed command: /b/ml-opt-devrel-x86-64-b1/build/bin/FileCheck -check-prefixes=GCN,GFX7 /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll
# .---command stderr------------
# | /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll:364:14: error: GFX7-NEXT: expected string not found in input
# | ; GFX7-NEXT: s_mul_i32 s0, s0, s2
# |              ^
# | <stdin>:425:9: note: scanning from here
# | ; %bb.0:
# |         ^
# | <stdin>:428:2: note: possible intended match here
# |  s_mul_i32 s0, s0, s1
# |  ^
# | /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll:439:14: error: GFX7-NEXT: is not on the line after the previous match
# | ; GFX7-NEXT: v_and_b32_e32 v0, 0xffff, v0
# |              ^
# | <stdin>:493:2: note: 'next' match was here
# |  v_and_b32_e32 v0, 0xffff, v0
# |  ^
# | <stdin>:490:41: note: previous match ended here
# |  s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
# |                                         ^
# | <stdin>:491:1: note: non-matching line after previous match is here
# |  v_lshrrev_b32_e32 v2, 16, v0
# | ^
# | 
# | Input file: <stdin>
# | Check file: /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/AMDGPU/GlobalISel/mul.ll
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |             .
# |             .
# |             .
# |           420:  .text 
# |           421:  .globl s_mul_v2i16 ; -- Begin function s_mul_v2i16 
# |           422:  .p2align 8 
# |           423:  .type s_mul_v2i16, at function 
# |           424: s_mul_v2i16: ; @s_mul_v2i16 
# |           425: ; %bb.0: 
# | next:364'0             X error: no match found
# |           426:  s_lshr_b32 s2, s0, 16 
...

```

</details>

https://github.com/llvm/llvm-project/pull/177314