[llvm] [GlobalISel][AArch64] Improve legalization of shift amounts (PR #208676)

[Ömer Sinan Ağacan via llvm-commits]

osa1 wrote:

@cofibrant it crashes without the patch. IR:

```
define i8 @test(i48 %a) {
entry:
  %b = lshr i48 %a, 15
  %c = trunc i48 %b to i8
  ret i8 %c
}
```

With `llc -stop-before=legalizer -global-isel`:

```
--- |
  ; ModuleID = 'lshr_crash.ll'
  source_filename = "lshr_crash.ll"
  target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-n32:64-S128-Fn32"
  
  define i8 @test(i48 %a) {
  entry:
    %b = lshr i48 %a, 15
    %c = trunc i48 %b to i8
    ret i8 %c
  }
...
---
name:            test
alignment:       4
exposesReturnsTwice: false
legalized:       false
regBankSelected: false
selected:        false
failedISel:      false
tracksRegLiveness: true
hasWinCFI:       false
noPhis:          false
isSSA:           true
noVRegs:         false
hasFakeUses:     false
callsEHReturn:   false
callsUnwindInit: false
hasEHContTarget: false
hasEHScopes:     false
hasEHFunclets:   false
isOutlined:      false
debugInstrRef:   false
failsVerification: false
tracksDebugUserValues: false
registers:
  - { id: 0, class: _, preferred-register: '', flags: [  ] }
  - { id: 1, class: _, preferred-register: '', flags: [  ] }
  - { id: 2, class: _, preferred-register: '', flags: [  ] }
  - { id: 3, class: _, preferred-register: '', flags: [  ] }
  - { id: 4, class: _, preferred-register: '', flags: [  ] }
  - { id: 5, class: _, preferred-register: '', flags: [  ] }
  - { id: 6, class: _, preferred-register: '', flags: [  ] }
  - { id: 7, class: _, preferred-register: '', flags: [  ] }
liveins:
  - { reg: '$x0', virtual-reg: '' }
frameInfo:
  isFrameAddressTaken: false
  isReturnAddressTaken: false
  hasStackMap:     false
  hasPatchPoint:   false
  stackSize:       0
  offsetAdjustment: 0
  maxAlignment:    1
  adjustsStack:    false
  hasCalls:        false
  framePointerPolicy: none
  stackProtector:  ''
  functionContext: ''
  maxCallFrameSize: 4294967295
  cvBytesOfCalleeSavedRegisters: 0
  hasOpaqueSPAdjustment: false
  hasVAStart:      false
  hasMustTailInVarArgFunc: false
  hasTailCall:     false
  isCalleeSavedInfoValid: false
  localFrameSize:  0
fixedStack:      []
stack:           []
entry_values:    []
callSites:       []
debugValueSubstitutions: []
constants:       []
machineFunctionInfo: {}
body:             |
  bb.1.entry:
    liveins: $x0
  
    %1:_(i64) = COPY $x0
    %0:_(i48) = G_TRUNC %1(i64)
    %6:_(i32) = G_TRUNC %0(i48)
    %2:_(i48) = G_CONSTANT i48 15
    %7:_(i32) = G_LSHR %6, %2(i48)
    $w0 = COPY %7(i32)
    RET_ReallyLR implicit $w0
...
```

If you run legalizer it crashes, so there's no output before this PR.

https://github.com/llvm/llvm-project/pull/208676