This reverts commit f7443905af1e06eaacda1e437fff8d54dc89c487.

This is to avoid an assertion if an undef operand appears in a
stackmap. This is important to avoid hitting verifier errors
when register allocation starts adding undefs in error scenarios.

Rather than trying to treat undef operands as special, leave them
alone and avoid producing an invalid spill. It would a bit more
precise to produce a spill of an undef register here, but that's not
exposed through the storeRegToStackSlot API.

https://reviews.llvm.org/D122605

This was an alternative to https://reviews.llvm.org/D122582

Identified with misc-include-cleaner.

Use a fixed width integer type and assert that DwarRegNum fits the 16
bits.

This is a follow up to review comments on #78600.

Reduce its size from 24 to 12 bytes. Improves memory consumption when
dealing with statepoint-heavy code.

The lists contain differences between register numbers, not the register
numbers themselves. Since a difference can also be negative, this also
changes its type to signed.

Changing the type to signed exposed a "bug". For AMDGPU, which has many
registers, the first element of a sequence could be as big as ~45k.
The value does not fit into int16_t, but fits into uint16_t. The bug
didn't show up because of unsigned wrapping and truncation of the Val
field in the advance() method.

To fix the issue, I changed the way regunit difflists are encoded. The
4-bit 'scale' field of MCRegisterDesc::RegUnit was replaced by 12-bit
number of the first regunit, and the first element of each of the lists
was removed. The higher 20 bits of RegUnit field contain the initial
offset into DiffLists array.
AMDGPU has 1'409 regunits (2^12 = 4'096), and the biggest offset is
80'041 (2^20 = 1'048'576). That is, there is enough room.

Changing the encoding method also resulted in a smaller array size, the
numbers are below (I omitted targets with less than 100 elements).

```
AMDGPU   | 80052 | 78741 |  -1,6%
RISCV    |  6498 |  6297 |  -3,1%
ARM      |  4181 |  3966 |  -5,1%
AArch64  |  2770 |  2592 |  -6,4%
PPC      |  1578 |  1441 |  -8,7%
Hexagon  |   994 |   740 | -25,6%
R600     |   508 |   398 | -21,7%
VE       |   471 |   459 |  -2,5%
Sparc    |   381 |   363 |  -4,7%
X86      |   326 |   208 | -36,2%
Mips     |   253 |   200 | -20,9%
SystemZ  |   186 |   162 | -12,9%
```

Reviewed By: foad, arsenm

Differential Revision: https://reviews.llvm.org/D151036

Differential Revision: https://reviews.llvm.org/D148613