[llvm] [RISCV][GISEL] Add legalizer for G_BSWAP (PR #70226)

[Michael Maitland via llvm-commits]

================
@@ -68,77 +68,32 @@ body:             |
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $x10
     ; CHECK-NEXT: [[COPY1:%[0-9]+]]:_(s32) = COPY $x11
----------------
michaelmaitland wrote:

I based my test on [`test/CodeGen/RISCV/GlobalISel/legalizer/rv32/legalize-abs.mir` ](https://github.com/llvm/llvm-project/pull/67577/files#diff-6a8b6e95c4cf7713102e96d1e9ffe39f8c25697449dea448c3216cb623443bf3). I don't see the MIR tracking those properties. Can you explain how you know that we need them here and why we didn't for the G_ABS patch?

I have added `tracksRegLiveness: true` manually to all of the test cases in this patch. This property does not cause any diff in autogenerated test CHECKs. I then took that file and ran it through llc with the options you suggest (and -stop-after=legalizer) which generates something like:

```
...
---
name:            bswap_i16
alignment:       1
exposesReturnsTwice: false
legalized:       true
regBankSelected: false
selected:        false
failedISel:      false
tracksRegLiveness: true
hasWinCFI:       false
callsEHReturn:   false
callsUnwindInit: false
hasEHCatchret:   false
hasEHScopes:     false
hasEHFunclets:   false
isOutlined:      false
debugInstrRef:   false
failsVerification: false
tracksDebugUserValues: false
registers:
  - { id: 0, class: _, preferred-register: '' }
  - { id: 1, class: _, preferred-register: '' }
  - { id: 2, class: _, preferred-register: '' }
  - { id: 3, class: _, preferred-register: '' }
  - { id: 4, class: _, preferred-register: '' }
  - { id: 5, class: _, preferred-register: '' }
  - { id: 6, class: _, preferred-register: '' }
  - { id: 7, class: _, preferred-register: '' }
  - { id: 8, class: _, preferred-register: '' }
  - { id: 9, class: _, preferred-register: '' }
  - { id: 10, class: _, preferred-register: '' }
  - { id: 11, class: _, preferred-register: '' }
  - { id: 12, class: _, preferred-register: '' }
  - { id: 13, class: _, preferred-register: '' }
  - { id: 14, class: _, preferred-register: '' }
  - { id: 15, class: _, preferred-register: '' }
  - { id: 16, class: _, preferred-register: '' }
  - { id: 17, class: _, preferred-register: '' }
  - { id: 18, class: _, preferred-register: '' }
  - { id: 19, class: _, preferred-register: '' }
  - { id: 20, class: _, preferred-register: '' }
  - { id: 21, class: _, preferred-register: '' }
  - { id: 22, class: _, preferred-register: '' }
liveins:         []
frameInfo:
  isFrameAddressTaken: false
  isReturnAddressTaken: false
  hasStackMap:     false
  hasPatchPoint:   false
  stackSize:       0
  offsetAdjustment: 0
  maxAlignment:    1
  adjustsStack:    false
  hasCalls:        false
  stackProtector:  ''
  functionContext: ''
  maxCallFrameSize: 4294967295
  cvBytesOfCalleeSavedRegisters: 0
  hasOpaqueSPAdjustment: false
  hasVAStart:      false
  hasMustTailInVarArgFunc: false
  hasTailCall:     false
  localFrameSize:  0
  savePoint:       ''
  restorePoint:    ''
fixedStack:      []
stack:           []
entry_values:    []
callSites:       []
debugValueSubstitutions: []
constants:       []
machineFunctionInfo:
  varArgsFrameIndex: 0
  varArgsSaveSize: 0
body:             |
  bb.0:
    liveins: $x10

    %0:_(s32) = COPY $x10
    %1:_(s32) = G_ASSERT_ZEXT %0, 16
    %15:_(s32) = G_CONSTANT i32 8
    %17:_(s32) = G_SHL %1, %15(s32)
    %12:_(s32) = G_CONSTANT i32 8
    %20:_(s32) = G_CONSTANT i32 65535
    %13:_(s32) = G_AND %1, %20
    %14:_(s32) = G_LSHR %13, %12(s32)
    %11:_(s32) = G_OR %14, %17
    %22:_(s32) = G_CONSTANT i32 65535
    %4:_(s32) = G_AND %11, %22
    $x10 = COPY %4(s32)
    PseudoRET implicit $x10
...
```
Do we really need to take any of these properties?

https://github.com/llvm/llvm-project/pull/70226