[Sema] NFC: Reorder misc bits for better code gen

[davezarzycki]

As a general rule, if one wants packs miscellaneous bits into what is
otherwise just a number, the low bits are preferable over the high bits,
at least on x86. On ppc64/ppc64le, the results are more of a wash. For
reference:

struct X {
unsigned kind : 3;
unsigned level : 29;
};

X example(X x, unsigned y) {
x.level += y;
return x;
}

bool example(X x) {
return x.level > 1;
}

==== x86_64 with 'kind' first ====

_Z7example1Xj:
leal (%rdi,%rsi,8), %eax
retq

_Z7example1X:
cmpl $15, %edi
seta %al
retq

==== x86_64 with 'kind' last ====

_Z7example1Xj:
leal (%rsi,%rdi), %eax
andl $536870911, %eax # imm = 0x1FFFFFFF
andl $-536870912, %edi # imm = 0xE0000000
orl %edi, %eax
retq

_Z7example1X:
testl $536870910, %edi # imm = 0x1FFFFFFE
setne %al
retq

==== PPC64 with 'kind' first ====

_Z7example1Xj:
add 5, 5, 4
rlwimi 5, 4, 0, 0, 2
stw 5, 0(3)
blr

_Z7example1X:
rlwinm 3, 3, 0, 3, 30
cntlzw 3, 3
srwi 3, 3, 5
xori 3, 3, 1
blr

==== PPC64 with 'kind' last ====

_Z7example1Xj:
slwi 5, 5, 3
add 4, 5, 4
stw 4, 0(3)
blr

_Z7example1X:
clrldi 3, 3, 32
subfic 3, 3, 15
rldicl 3, 3, 1, 63
blr

==== PPC64LE with 'kind' first ====

_Z7example1Xj:
slwi 4, 4, 3
add 3, 4, 3
blr

_Z7example1X:
clrldi 3, 3, 32
subfic 3, 3, 15
rldicl 3, 3, 1, 63
blr

==== PPC64LE with 'kind' last ====

_Z7example1Xj:
add 4, 4, 3
rlwimi 3, 4, 0, 3, 31
blr

_Z7example1X:
rlwinm 3, 3, 0, 3, 30
cntlzw 3, 3
srwi 3, 3, 5
xori 3, 3, 1
blr

[davezarzycki]

@swift-ci please smoke test