[NFC] Adding test for clobbering of high registers in Thumb

[pratlucas]

Summary

Prior to the changes from D52010, clobbering Thumb's high registers in
inline asm would cause incorrect code to be generated - or an assertion
failure for debug builds. Now that the issue is no longer reproducible,
this patch adds a MIR test to cover that scenario.

[arsenm]

llvm/test/CodeGen/Thumb/high-reg-clobber.mir

7–22

You can drop the IR section

[pratlucas]

Dropping test's IR section.

[pratlucas]

Ping.

[arsenm]

LGTM

llvm/test/CodeGen/Thumb/high-reg-clobber.mir

12–17

Can drop this too

llvm/test/CodeGen/Thumb/high-reg-clobber.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple thumbv6m-arm-none-eabi -run-pass regallocbasic %s -o - | FileCheck %s
+# RUN: llc -mtriple thumbv6m-arm-none-eabi -run-pass greedy %s -o - | FileCheck %s
+# RUN: llc -mtriple thumbv6m-arm-none-eabi -run-pass regallocfast %s -o - | FileCheck %s --check-prefix=FAST
+
+...
+---
+name:            constraint_h
+alignment:       2
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: tgpr }
+  - { id: 1, class: hgpr }
+  - { id: 2, class: tgpr }
+liveins:
+  - { reg: '$r0', virtual-reg: '%0' }
+frameInfo:
+  maxAlignment:    4
+  maxCallFrameSize: 0
+  localFrameSize:  4
+stack:
+  - { id: 0, size: 4, alignment: 4, local-offset: -4 }
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    liveins: $r0
+
+    ; CHECK-LABEL: name: constraint_h
+    ; CHECK: liveins: $r0
+    ; CHECK: [[COPY:%[0-9]+]]:tgpr = COPY $r0
+    ; CHECK: tSTRspi [[COPY]], %stack.0, 0, 14 /* CC::al */, $noreg
+    ; CHECK: [[tLDRspi:%[0-9]+]]:tgpr = tLDRspi %stack.0, 0, 14 /* CC::al */, $noreg
+    ; CHECK: [[COPY1:%[0-9]+]]:hgpr = COPY [[tLDRspi]]
+    ; CHECK: INLINEASM &"mov r12, $0", 1 /* sideeffect attdialect */, 1048585 /* reguse:GPRnoip_and_GPRwithAPSR_NZCVnosp */, [[COPY1]], 12 /* clobber */, implicit-def early-clobber $r12
+    ; CHECK: tBX_RET 14 /* CC::al */, $noreg
+    ; FAST-LABEL: name: constraint_h
+    ; FAST: liveins: $r0
+    ; FAST: tSTRspi killed renamable $r0, %stack.0, 0, 14 /* CC::al */, $noreg
+    ; FAST: renamable $r0 = tLDRspi %stack.0, 0, 14 /* CC::al */, $noreg
+    ; FAST: renamable $r8 = COPY killed renamable $r0
+    ; FAST: INLINEASM &"mov r12, $0", 1 /* sideeffect attdialect */, 1048585 /* reguse:GPRnoip_and_GPRwithAPSR_NZCVnosp */, killed renamable $r8, 12 /* clobber */, implicit-def dead early-clobber $r12
+    ; FAST: tBX_RET 14 /* CC::al */, $noreg
+    %0:tgpr = COPY $r0
+    tSTRspi %0, %stack.0, 0, 14 /* CC::al */, $noreg
+    %2:tgpr = tLDRspi %stack.0, 0, 14 /* CC::al */, $noreg
+    %1:hgpr = COPY %2
+    INLINEASM &"mov r12, $0", 1 /* sideeffect attdialect */, 1048585 /* reguse:hGPR */, %1, 12 /* clobber */, implicit-def early-clobber $r12
+    tBX_RET 14 /* CC::al */, $noreg
+
+...